mbproxy: Wave 3 cleanups, docs, and test gaps from 2026-05-14 review

Closes the Wave 3 (cleanup) tier of codereviews/2026-05-14/RemediationPlan.md.
Tests: 378 pass / 0 fail (baseline 370 + 8 new W3 regression tests).

Code cleanups:
  * PlcMultiplexer: removed dead `elapsedMs` calculation (the actual EWMA
    conversion uses Stopwatch ticks two lines below).
  * UpstreamPipe.FillAsync: dropped the meaningless `firstRead && remaining
    == count ? false : false` ternary; both branches were `false`.
  * InFlightByKeyMap.TryAttachOrCreate (always returned `true`) renamed to
    `AttachOrCreate` and made `void`. Test sites updated to drop the dead
    `bool ok = ...; ok.ShouldBeTrue();` assertions.
  * BcdCodec.HasBadNibble promoted from private to internal; the duplicate
    copy in BcdPduPipeline removed and the call sites updated to
    `BcdCodec.HasBadNibble`.
  * PlcMultiplexer watchdog comment fixed: said "1-second floor", code uses
    100 ms. Now both agree.
  * StatusSnapshotBuilder: simplified the unreachable
    `RemoteEp?.ToString() ?? RemoteEp?.Address.ToString() ?? "?"` to
    `RemoteEp?.ToString() ?? "?"`.
  * Mbproxy.csproj: stale "deferred" Polly comment replaced with a real
    description of where Polly is used (BackendConnect + ListenerRecovery).

Doc updates:
  * README: added a callout about the unconventional 32-bit BCD wire format
    ("two base-10000 digits in CDAB", not standard binary CDAB Int32) so
    integrators using off-the-shelf clients learn about the silent-corruption
    hazard before configuring writes.
  * docs/design.md: clarified `cacheMissCount` and `coalescedMissCount`
    semantics — "miss" means "did not find a fresh entry / did not coalesce",
    NOT "produced a backend round-trip". Operators wanting actual backend
    traffic should compute `miss − coalescedHit − exception04`.
  * docs/Architecture/ResponseCache.md: documented the structural
    "skip invalidation while recovering" gating (no backend reader during
    recovery → no FC06/FC16 response → no invalidation).
  * docs/Operations/Configuration.md: noted that the Event Log sink is the
    custom EventLogBridge, not Serilog.Sinks.EventLog (W2.23 cached check).
  * docs/plan/README.md: added a Phase 12 row pointing at the remediation
    plan and linking out to codereviews/2026-05-14/.

Test additions (W3 high-value gaps):
  * BcdPduPipelineTests:
    - FC16_WriteStartsOnHighWord_Of32BitPair_PassesThroughRaw_WithPartialWarning
      (symmetric inverse of the existing low-side partial-overlap test).
    - FC03_Mixed_16Bit_32Bit_AndNonBcd_InOneRead_OnlyConfiguredSlotsRewritten
      (mixed-slot routing in a single FC03 read).
    - FC16_Response_PassesThroughUnchanged_RegardlessOfTagMap (FC16 response
      carries no register data; rewriter must pass through).
  * AdminEndpointTests:
    - NonGetMethod_AgainstAdminRoutes_Returns405 (Theory: POST/PUT/DELETE/
      PATCH against `/` and `/status.json` must return 405; guards against
      an accidental MapPost being added later).
  * HotReloadE2ETests:
    - E2E_TagListReload_OnCacheablePlc_EmitsCacheFlushedEvent (validates the
      W2.8 cache.flushed wiring end-to-end via the real FileSystemWatcher
      reload path).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mbproxy/docs/design.md

@@ -136,7 +136,7 @@ Properties:
 - **Hot-reloadable on/off.** `Mbproxy.Resilience.ReadCoalescing.Enabled` defaults to `true`. Flipping it to `false` at runtime leaves running coalesced entries to drain naturally; subsequent FC03/04 requests take the Phase-9 (one round-trip per upstream request) path.
 - **Transparency contract preserved.** Each upstream client still sees its own original MBAP TxId on the response. The BCD rewriter runs once on the shared response buffer; per-party copies are only made when fan-out has more than one party.
 
-Counter accounting balance (per snapshot): `coalescedHitCount + coalescedMissCount` equals the total FC03 + FC04 requests seen since the multiplexer was constructed. Both counters increment regardless of whether the coalescing feature is enabled — `coalescedHitCount` is 0 when disabled, but every read still increments `coalescedMissCount`.
+Counter accounting balance (per snapshot): `coalescedHitCount + coalescedMissCount` equals the total FC03 + FC04 requests seen since the multiplexer was constructed. Both counters increment regardless of whether the coalescing feature is enabled — `coalescedHitCount` is 0 when disabled, but every read still increments `coalescedMissCount`. **Saturation paths (allocator full, duplicate-key race) also count as a miss** even though they produce no backend round-trip — the identity above is preserved by counting every entry into the coalescing path, not every backend send. Operators wanting "actual backend round-trips opened" should subtract the multiplexer's exception-04 frames produced from saturation.
 
 ## Response cache (Phase 11) — opt-in bounded-staleness cache
 
@@ -173,7 +173,7 @@ The BCD rewriter runs once on the cache-miss path (the backend reader task decod
 ### Counter accounting
 
 - `cacheHitCount` — FC03/FC04 requests served from the cache.
-- `cacheMissCount` — FC03/FC04 requests that fell through to the coalescing/backend path. (Cache hit + Cache miss = total FC03/FC04 requests that were cache-eligible, i.e. whose resolved TTL was > 0; reads whose effective TTL is 0 increment neither.)
+- `cacheMissCount` — FC03/FC04 requests that fell through to the coalescing/backend path. (Cache hit + Cache miss = total FC03/FC04 requests that were cache-eligible, i.e. whose resolved TTL was > 0; reads whose effective TTL is 0 increment neither.) **A "miss" does NOT mean "produced a backend round-trip."** Two upstream peers issuing the same cache-eligible read both increment `cacheMissCount`; one then opens a backend round-trip and the other coalesces onto it via the InFlightByKey path (incrementing `coalescedHitCount`). Operators reading these counters as "backend reads opened" should use `cacheMissCount − coalescedHitCount` as the lower bound on actual backend traffic.
 - `cacheInvalidations` — count of cache entries invalidated by FC06/FC16 write responses.
 - `cacheEntryCount` — point-in-time snapshot of `ResponseCache.Count` (Tier-2 memory-watch KPI).
 - `cacheBytes` — point-in-time approximation of cached PDU bytes (Tier-2 memory-watch KPI).