mbproxy: add in-flight read coalescing (Phase 10)

When two or more upstream clients send the same FC03/FC04 read while a
matching request is already in flight on the same PLC's multiplexed
backend socket, attach the late arrivals to the existing InFlightRequest
.InterestedParties list instead of opening a second backend round-trip.
The single backend response fans out to every attached party with each
party's original MBAP TxId restored individually. Zero post-response
staleness — coalescing operates entirely within the in-flight window
(microseconds to ~10 ms typical); the proxy is NOT a cache layer.

Headline mechanism:

- New record struct CoalescingKey(UnitId, Fc, StartAddress, Qty) keys
  the per-PLC InFlightByKeyMap. FC03 and FC04 are separate Modbus
  tables and never share a key; different unit IDs never coalesce;
  writes (FC06/FC16) bypass the coalescing path entirely.
- InFlightByKeyMap uses a simple lock around a Dictionary; atomic
  TryAttachOrCreate either appends a new party to the in-flight
  request's mutable List<InterestedParty> or invokes a factory to
  build a fresh entry. Per-entry MaxParties cap (default 32) bounds
  fan-out cost; past the cap, the next arrival opens a new entry.
- PlcMultiplexer.OnUpstreamFrameAsync takes the coalescing path for
  FC03/FC04 when Mbproxy.Resilience.ReadCoalescing.Enabled. The
  factory closure does the Phase-9 work (allocate TxId, add to
  CorrelationMap); the channel send happens AFTER returning from
  TryAttachOrCreate so the map lock is not held across the async send.
- Response fan-out in RunBackendReaderAsync removes the entry from
  InFlightByKeyMap before iterating InterestedParties, ensuring no
  concurrent attach can mutate the list during iteration.
- Cascade + watchdog paths also drain the key map so a stale entry
  cannot outlive its backend round-trip.

Counter accounting balance (per snapshot): CoalescedHitCount +
CoalescedMissCount equals total FC03 + FC04 requests since startup.
Even with coalescing disabled, every read still bumps Miss so dashboard
math stays balanced.

New surface (additive only):
- src/Mbproxy/Proxy/Multiplexing/CoalescingKey.cs
- src/Mbproxy/Proxy/Multiplexing/InFlightByKeyMap.cs
- src/Mbproxy/Proxy/Multiplexing/CoalescingLogEvents.cs
- ReadCoalescingOptions on ResilienceOptions
- CoalescedHitCount / CoalescedMissCount /
  CoalescedResponseToDeadUpstream counters surfaced on /status.json
  per PLC and as a compact "Coal" cell on the HTML status page.

Phase 9 test patch: TwoUpstreams_ProxyTxIds_AreDistinct_OnTheWire
previously read the same register from both clients (which now
coalesces). Patched to read two different addresses so the test still
proves distinct backend TxIds without violating the coalescing
contract.

Tests added: 24 new (19 unit + 5 E2E):
- CoalescingKeyTests (5)
- InFlightByKeyMapTests (6, includes concurrent stress)
- ReadCoalescingTests (8, stub-backend with deterministic delay)
- ReadCoalescingE2ETests (5, pymodbus simulator; coalescing-active
  during overlap is proven against the stub, not the sim, due to
  pymodbus 3.13's known concurrent-frame bug)

Total: 325 tests passing (282 unit + 43 E2E).

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

mbproxy/docs/kpi.md

@@ -23,7 +23,7 @@ For context — every recommended addition below is *in addition to* this list.
 | Per-PLC listener | `state`, `lastBindError`, `recoveryAttempts` |
 | Per-PLC clients | `connected`, `remoteEndpoints[]` (remote, connectedAtUtc, pdusForwarded) |
 | Per-PLC PDUs | `forwarded`, `byFc.{fc03,fc04,fc06,fc16,other}`, `rewrittenSlots`, `partialBcdWarnings` |
-| Per-PLC backend | `connectsSuccess`, `connectsFailed`, `exceptionsByCode.{code01..code04}`, `lastRoundTripMs` |
+| Per-PLC backend | `connectsSuccess`, `connectsFailed`, `exceptionsByCode.{code01..code04}`, `lastRoundTripMs`, `inFlight`, `maxInFlight`, `txIdWraps`, `disconnectCascades`, `queueDepth`, `coalescedHitCount`, `coalescedMissCount`, `coalescedResponseToDeadUpstream` |
 | Per-PLC bytes | `upstreamIn`, `upstreamOut` |
 
 Counters are **cumulative since process start**. A restart resets them.
@@ -112,16 +112,16 @@ The proxy holds one backend socket per PLC and multiplexes upstream clients via
 
 **Why this matters.** Multiplexing concentrates connection risk: a single backend disconnect now cascades to every attached upstream client. The cascade counter quantifies that blast radius. Queue depth is the new latency leading indicator (today's `lastRoundTripMs` measures wire latency only; queue depth reveals proxy-side backlog).
 
-### 1.7 Read coalescing — **[requires Phase 10](plan/10-read-coalescing.md)**
+### 1.7 Read coalescing — **shipped in [Phase 10](plan/10-read-coalescing.md)**
 
-After Phase 10 ships, same-key FC03/04 reads within the in-flight window attach to one another instead of generating duplicate backend requests. The coalescing ratio is the headline metric.
+Same-key FC03/04 reads within the in-flight window attach to one another instead of generating duplicate backend requests. The coalescing ratio is the headline metric. `coalescedHitCount + coalescedMissCount` equals total FC03/04 request count per snapshot — the math always balances.
 
 | KPI | Definition | Source | Widget | Alert | Effort |
 |-----|------------|--------|--------|-------|--------|
 | `backend.coalescedHitCount` | FC03/04 requests attached to an already-in-flight peer | Phase-10 counter | Sparkline | None — trend-watch | (in Phase 10 scope) |
 | `backend.coalescedMissCount` | FC03/04 requests that created a fresh backend round-trip | Phase-10 counter | Sparkline | None — trend-watch | (in Phase 10 scope) |
 | `backend.coalescingRatio` | `Hit / (Hit + Miss)` over the trailing window | Derived (dashboard) | Stat tile per PLC | None; a low ratio just means clients aren't synchronised on the same registers — informational | (in Phase 10 scope) |
-| `backend.coalescedResponseToDeadUpstream` | Fan-out responses dropped because the attached upstream disconnected mid-flight | Phase-10 counter | Stat tile per PLC | Spike → client churn during traffic burst; usually not actionable | (in Phase 10 scope) |
+| `backend.coalescedResponseToDeadUpstream` | Fan-out responses dropped because the attached upstream disconnected mid-flight | Phase-10 counter | Stat tile per PLC | Spike → client churn during traffic burst; usually not actionable (Tier 2 priority) | (in Phase 10 scope) |
 
 **Why this matters.** Coalescing-ratio is the "how much PLC traffic did we save" metric. A 60% ratio means 60% of FC03/04 reads landed on an existing in-flight request — that's roughly 60% reduction in backend PDU rate vs the pre-Phase-10 model. The dead-upstream counter is a churn indicator that's invisible in any other metric.