perf(historian): index the QuestDB sample tag column; read the data range before the buckets (#683)

QuestDB: {p}sample.tag is now SYMBOL INDEX. The processed-read data-range
lookups (EarliestSample / LatestSample) were partition scans of the unindexed
symbol; on 53.6M rows / 120 tags / 61 daily partitions the indexed seeks are
1-1.5 ms warm / 2-4 ms cold whatever the tag's history (unindexed: up to 11 ms
warm, 14 ms cold; cost grew with the table). The bootstrapper adds the index to a
pre-#683 table (ALTER ... ADD INDEX, a WAL txn, idempotent, ~3 s for 68M rows);
AutoCreateSchema=false only logs the statement. Disk +~12%; write apply time
within noise; main per-sample/raw/time-weighted queries unchanged or faster.

Rejected by measurement/analysis: bounded existence probes (still full scans
when a side is absent), LATEST ON (slowest unindexed), per-tag first/last side
table (cannot track TTL drops or deletes INSERT-only).

Both backends now read the data range before the bucket statement: an append
racing the read reaches the buckets but not the range, so it can no longer
produce a Good Count of 0 for an interval that has samples.

Tests: TagIndexTests (new-table index, bound-param seek uses the index,
tombstones excluded / outage marker counted, pre-#683 migration with identical
processed results across 10 aggregates x 4 windows x both directions).

CLAUDE.md

@@ -1350,7 +1350,7 @@ historian tagname, which is the tag's driver `FullName`.
 ### Pluggable backends, SQL Server and HistoryUpdate (2026-10)
 
 - **Selection:** `ServerHistorian:Backend` (`Gateway` default | `SqlServer` | `QuestDb`; unknown fails host start). `HistorianBackendRegistry` → `IHistorianBackendProvider` builds all five seams; the gateway keys are only required for `Gateway`. `SqlServer` uses `ServerHistorian:SqlServer:{ConnectionString (required), Schema=hist, CommandTimeout=30s, AutoCreateSchema=true}`.
-- **SQL Server** (`Driver.Historian.SqlServer`): `hist.Sample` PK `(TagId, SourceTimeUtc)` — writes are insert-if-absent, so outbox redelivery is idempotent. Stores every value kind with its **real status**; per-sample aggregates (10, simple definitions) are pushed down to T-SQL with bucket math in **100 ns ticks**; the Part 13 time-weighted seven (TimeAverage/TimeAverage2/Total/Total2/Interpolative, #671 — ⚠️ `Total` is now the time-weighted integral, no longer `SUM` — plus the **duration-based** PercentGood/PercentBad, #680) stream window + 4 bound rows into the shared `TimeWeightedAggregator` (both SQL backends, identical results; reverse `Interpolative` anchored at the later time; window capped by `ServerHistorian:MaxTimeWeightedRawRows` ⇒ `BadTooManyOperations`, percentages included). ⚠️ **#680 status rules (client-visible):** TimeAverage/Total are Uncertain if any part of the interval uses an Uncertain value, is bridged across a Bad sample, is extrapolated past the last sample (⇒ a live tag's latest interval is Uncertain) or is not covered; TimeAverage2/Total2 keep the SDK time-based rule (Uncertain time counts as Good) but never extrapolate past the last sample — that interval is Partial, later ones `BadNoData` (a report-by-exception tag with nothing after its last sample reads `BadNoData` until its next sample arrives). `docs/Historian.md` §"Time-weighted aggregates". ⚠️ **#680 per-sample (client-visible):** `Count` counts **Good** samples only (value-based status; an all-Bad interval is generic Bad, value null) and an empty interval **inside the tag's data range** is `0`/Good (outside it still `BadNoData`); `Start`/`End` return the **raw** first/last sample of any quality — its own timestamp and status, value null when Bad; `Delta` is Uncertain only when the interval's first or last sample is unusable; ⚠️ #684: `Average`/`Minimum`/`Maximum`/`Range`/`*ActualTime` use **Good samples only** (was Good + Uncertain; an Uncertain-only interval is `BadNoData`), Min/Max-family status Uncertain on a Bad sample, an Uncertain value beyond the Good extreme (Part 13 — the SDK ignores that) or an Uncertain non-numeric sample, and only a **Good** String/DateTime sample refuses them (`BadAggregateInvalidInputs`; Delta still refuses any usable one); and every per-sample aggregate but `Average` sets **end-of-data Partial** on the interval holding the last sample (forward incl. its earlier edge, reverse strictly inside), never on a Bad result. The data-range lookup is a seek on SQL Server but a partition scan on QuestDB (unindexed `tag` SYMBOL). `docs/Historian.md` §"Aggregates". Docker-dev recipe, schema and aggregate semantics: `docs/Historian.md` §"SQL Server backend". Integration tests need `HISTORIAN_SQL_TEST_CONNECTION`. **Retention (#667):** opt-in `SqlServer:RetentionDays` (+ `PurgeInterval` 1h, `PurgeBatchSize` 4000) -> `HistorianRetentionService` purges old `hist.Sample` (per tag) / `hist.Event` in `DELETE TOP(n)` batches, Primary-gated on `IHistorizationRoleView`, one purger per DB via `sp_getapplock`; counter `otopcua.historian.retention.purged{table}`. Optional `IHistorianBackendProvider.CreateRetention` (default null). `docs/Historian.md` §Retention.
+- **SQL Server** (`Driver.Historian.SqlServer`): `hist.Sample` PK `(TagId, SourceTimeUtc)` — writes are insert-if-absent, so outbox redelivery is idempotent. Stores every value kind with its **real status**; per-sample aggregates (10, simple definitions) are pushed down to T-SQL with bucket math in **100 ns ticks**; the Part 13 time-weighted seven (TimeAverage/TimeAverage2/Total/Total2/Interpolative, #671 — ⚠️ `Total` is now the time-weighted integral, no longer `SUM` — plus the **duration-based** PercentGood/PercentBad, #680) stream window + 4 bound rows into the shared `TimeWeightedAggregator` (both SQL backends, identical results; reverse `Interpolative` anchored at the later time; window capped by `ServerHistorian:MaxTimeWeightedRawRows` ⇒ `BadTooManyOperations`, percentages included). ⚠️ **#680 status rules (client-visible):** TimeAverage/Total are Uncertain if any part of the interval uses an Uncertain value, is bridged across a Bad sample, is extrapolated past the last sample (⇒ a live tag's latest interval is Uncertain) or is not covered; TimeAverage2/Total2 keep the SDK time-based rule (Uncertain time counts as Good) but never extrapolate past the last sample — that interval is Partial, later ones `BadNoData` (a report-by-exception tag with nothing after its last sample reads `BadNoData` until its next sample arrives). `docs/Historian.md` §"Time-weighted aggregates". ⚠️ **#680 per-sample (client-visible):** `Count` counts **Good** samples only (value-based status; an all-Bad interval is generic Bad, value null) and an empty interval **inside the tag's data range** is `0`/Good (outside it still `BadNoData`); `Start`/`End` return the **raw** first/last sample of any quality — its own timestamp and status, value null when Bad; `Delta` is Uncertain only when the interval's first or last sample is unusable; ⚠️ #684: `Average`/`Minimum`/`Maximum`/`Range`/`*ActualTime` use **Good samples only** (was Good + Uncertain; an Uncertain-only interval is `BadNoData`), Min/Max-family status Uncertain on a Bad sample, an Uncertain value beyond the Good extreme (Part 13 — the SDK ignores that) or an Uncertain non-numeric sample, and only a **Good** String/DateTime sample refuses them (`BadAggregateInvalidInputs`; Delta still refuses any usable one); and every per-sample aggregate but `Average` sets **end-of-data Partial** on the interval holding the last sample (forward incl. its earlier edge, reverse strictly inside), never on a Bad result. The data-range lookup is a key seek on SQL Server and a `tag` symbol-index seek on QuestDB (#683 — `{p}sample.tag` is `SYMBOL INDEX`; the bootstrapper ADDs it to a pre-#683 table, idempotent), read BEFORE the bucket statement so a racing append can never yield a Good `Count` of 0. `docs/Historian.md` §"Aggregates". Docker-dev recipe, schema and aggregate semantics: `docs/Historian.md` §"SQL Server backend". Integration tests need `HISTORIAN_SQL_TEST_CONNECTION`. **Retention (#667):** opt-in `SqlServer:RetentionDays` (+ `PurgeInterval` 1h, `PurgeBatchSize` 4000) -> `HistorianRetentionService` purges old `hist.Sample` (per tag) / `hist.Event` in `DELETE TOP(n)` batches, Primary-gated on `IHistorizationRoleView`, one purger per DB via `sp_getapplock`; counter `otopcua.historian.retention.purged{table}`. Optional `IHistorianBackendProvider.CreateRetention` (default null). `docs/Historian.md` §Retention.
 - **QuestDB** (`Driver.Historian.QuestDb`, `ServerHistorian:QuestDb:*`, Npgsql over PG wire; docker-dev overlay `docker-compose.questdb.yml`): same kinds/status/aggregates/HistoryUpdate as SqlServer, soft delete by tombstone (INSERT only — never `UPDATE`/`DELETE`; every read filters `NOT deleted`). ⚠️ **Timestamps are µs** (`QuestDbConnection.Round`/`BindTimestamp`; `IHistorianDataSource.TimestampResolutionTicks` = 10 steps HistoryRead paging), **pre-1970 cannot be stored** (writer drops, HistoryUpdate `BadOutOfRange`), every write waits for WAL visibility, TTL is enforced on write commits only, set Npgsql `Maximum Pool Size` < QuestDB's 64. `docs/Historian.md` §"QuestDB backend".
 - ⚠️ **Reverse processed reads are Part 13/SDK-anchored (#677, client-visible):** every aggregate on SqlServer/QuestDB now anchors a reverse read at its later time — window `(end, start]`, partial interval earliest, each stamped with its later edge (was: earlier-anchored, stamped earlier edge); on forward reads Partial also marks the interval the data starts inside (not Average — the SDK's rule, forward-only); a non-default client `AggregateConfiguration` ⇒ `BadAggregateConfigurationRejected`; Gateway provisioning sends `IntegralDivisor = 1`. SDK-parity tests: `SdkAggregateParityTests`. `docs/Historian.md` §Aggregates.
 - **Advertised aggregates (#682):** `IHistorianDataSource.SupportedAggregates` (Gateway 4 via `GatewayHistorianDataSource.ServedAggregates`, SqlServer/QuestDB 17 via `AggregateFamilies.Served`, Null **none**; a null set ⇒ all) is the one authority for `HistoryServerCapabilities/AggregateFunctions` (i=11201 — only its folder references are trimmed) and the up-front `BadAggregateNotSupported` in HistoryRead-Processed. ⚠️ `ServerCapabilities/AggregateFunctions` (i=2997) is deliberately NOT trimmed: it keeps the SDK's 37 because live `AggregateFilter` subscriptions still compute them all. ⚠️ Historian disabled ⇒ Processed is now `BadAggregateNotSupported`, not `GoodNoData`. `docs/Historian.md` §Advertised aggregates.