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).

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.QuestDb/QuestDbTables.cs

@@ -39,9 +39,15 @@ internal static class QuestDbTables
     /// </summary>
     public static string[] Expiring(string prefix) => [Sample(prefix), Event(prefix), SampleModified(prefix)];
 
-    /// <summary><c>CREATE TABLE IF NOT EXISTS</c> for the sample table.</summary>
+    /// <summary>
+    ///     <c>CREATE TABLE IF NOT EXISTS</c> for the sample table. <c>tag</c> carries a symbol (bitmap) index (#683): every
+    ///     read filters on one tag, and without the index QuestDB answers a <c>tag = x</c> seek by scanning the tag column of
+    ///     every partition in the time range — the data-range lookups (<see cref="QuestDbAggregateQueries.EarliestSample"/> /
+    ///     <c>LatestSample</c>) of a stopped or late-starting tag then scan every partition on one side of the window.
+    ///     A table created before #683 gains the index through <see cref="AddTagIndex"/>.
+    /// </summary>
     public static string CreateSample(string prefix, int? retentionDays) =>
-        $"CREATE TABLE IF NOT EXISTS {Sample(prefix)} (ts TIMESTAMP, tag SYMBOL, kind BYTE, num_value DOUBLE, " +
+        $"CREATE TABLE IF NOT EXISTS {Sample(prefix)} (ts TIMESTAMP, tag SYMBOL INDEX, kind BYTE, num_value DOUBLE, " +
         "int_value LONG, str_value VARCHAR, status LONG, deleted BOOLEAN, server_ts TIMESTAMP) " +
         $"TIMESTAMP(ts) PARTITION BY DAY{Ttl(retentionDays)} WAL DEDUP UPSERT KEYS(ts, tag)";
 
@@ -74,6 +80,21 @@ internal static class QuestDbTables
         [CreateSample(prefix, retentionDays), CreateEvent(prefix, retentionDays), CreateTag(prefix),
             CreateSampleModified(prefix, retentionDays)];
 
+    /// <summary>
+    ///     <c>ALTER TABLE t ALTER COLUMN tag ADD INDEX</c> — the #683 migration of a sample table created without the tag
+    ///     index. A WAL transaction (visible once applied); re-issuing it on an indexed column is a harmless no-op (the table
+    ///     is not suspended), so two nodes booting together may both send it.
+    /// </summary>
+    public static string AddTagIndex(string table) => $"ALTER TABLE {table} ALTER COLUMN tag ADD INDEX";
+
+    /// <summary>
+    ///     Whether <paramref name="table"/>'s <c>tag</c> column is indexed: one row, one BOOLEAN column, no row when the
+    ///     table or column is missing. The name is inlined (<c>table_columns()</c> takes a constant); it is always a
+    ///     prefix-validated name built here.
+    /// </summary>
+    public static string TagIndexedQuery(string table) =>
+        $"SELECT indexed FROM table_columns('{table}') WHERE \"column\" = 'tag'";
+
     /// <summary><c>ALTER TABLE t SET TTL n DAYS</c>; <c>null</c> removes the TTL (<c>SET TTL 0 DAYS</c>).</summary>
     public static string AlterTtl(string table, int? retentionDays) =>
         $"ALTER TABLE {table} SET TTL {(retentionDays ?? 0).ToString(CultureInfo.InvariantCulture)} DAYS";