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/Architecture/ResponseCache.md

@@ -303,6 +303,17 @@ cached read is still consistent with the device's actual state. Skipping
 the invalidation matches reality — the write did not take effect, so the
 read is not stale.
 
+The skip is **structural**, not conditional. Cache invalidation only
+fires inside the per-PLC backend reader task, after a non-exception
+FC06/FC16 response arrives from the PLC. A `recovering` supervisor has
+torn down its multiplexer and there is no backend reader, so no response
+can land and the invalidation path is never entered. This is the
+reasoning the code at `Proxy/Multiplexing/PlcMultiplexer.cs` documents
+inline (W2.9). If a future change ever produced a write response off the
+live backend (e.g. a mocked-response path), an explicit `Recovering`
+check would need to be added at the invalidator call site to keep the
+skip semantics correct.
+
 ## No Persistence
 
 The cache is purely in-memory. Process restart wipes every entry. There