fix: don't advance the book-version chain when no frame is emitted (#115)

[emrebulutlar]

Fixes #115.

Root cause

MarketPublisher.encodeBookSnapshot advanced lastPublishedBookVersion = collectedBookVersion unconditionally — even when encodeBookDelta returned 0 because the mutation happened below the visible top-20 depth (side version bumped, visible diff empty, nothing broadcast). The next visible delta then carried a fromVersion no consumer ever received, which the gateway correctly treats as a chain break (#96): book flagged stale, frozen until the next periodic resnapshot (#112).

This explains every piece of the live evidence:

• side-specific missing versions (deltaBidV == haveBidV, ask versions skipping by 4) — deep ask-side churn from the sim;
• match_dropped_market_msgs_total = 0 on all nodes — nothing was dropped, nothing was ever sent;
• steady ~4 breaks/market/min in steady state — the rate of below-depth-20 churn;
• "exactly one break per market right after the initial snapshot" (fix: chain-break diagnostics at default log level with per-side versions #114 diagnostics).

No raw-frame capture needed: the defect reproduces deterministically in a unit test (see below), failing with the exact live shape (delta from=2000 while the consumer holds 1000).

Fix

Advance lastPublishedBookVersion only when a frame was actually encoded. An unsent version gap is fine: the next delta spans it (visible state → visible state), so the emitted stream is chain-continuous by construction. Genuine shed (bounded-queue drop, counted) still breaks the chain at the gateway — which is the correct signal, recovered by the #112 resnapshot.

Audit finding fixed alongside (same family)

AppClusteredService.drainQueue gives up on a message for a session after 3 backpressured session.offer retries with no accounting anywhere — an egress loss invisible to every existing drop counter. Now counted as match_egress_offer_giveups_total (should stay 0) with a rate-limited CRITICAL log. Retry-policy hardening (esp. for the reliable OMS queue) left as a follow-up.

Tests

• MarketPublisherBookChainTest.deepBookChangeMustNotBreakTheEmittedVersionChain — snapshot → deep-only change (no frame, by design) → visible change → delta must chain from the last emitted frame's version. Fails pre-fix with expected:<1000> but was:<2000>.
• MarketPublisherBookChainTest.emittedStreamIsChainContinuousAcrossManyMixedFlushes — chain continuity across 10 interleaved deep/visible flush cycles.
• Full match-cluster suite: 454 tests green.

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