Wire V2 promotion into the real master failover decision path:
promoteReplica() now dispatches to promoteReplicaV2() when
blockV2Promotion flag is true. V2 path queries each candidate for
fresh evidence via pluggable BlockPromotionEvidenceQuerier, selects
by CommittedLSN (durability-first), and fail-closes when no eligible
candidate exists. No silent fallback to V1.
Feature flag: blockV2Promotion bool on MasterServer. When false,
existing promoteReplicaV1() (health-score-first) is used unchanged.
Flag is explicit and observable, not a hidden rescue path.
Registry: add PromoteReplicaByServer() for V2 path where master
already knows the winner. Clear stale EngineProjectionMode in
applyPromotionLocked (complements T1 turnover fix).
T2 fix: fail-closed when V2 core projection is absent —
Eligible=false with reason "missing_engine_projection". CommittedLSN
from core used unconditionally (no WALHeadLSN overstatement).
5 T3 integration tests: higher CommittedLSN wins, all-ineligible
fail-closed, evidence-failure fail-closed, flag-off uses legacy,
epoch bump + assignment enqueue only after selection.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
VS-side evidence handler (QueryBlockPromotionEvidence) reads live
blockvol.Status() + V2 core projection at call time. Fail-closed:
no core projection → ineligible with reason "missing_engine_projection".
Engine CommittedLSN used unconditionally when core present (no WALHeadLSN
overstatement). Eligibility owned by local V2 engine, not master.
Master-side selection (selectDurabilityFirstCandidate): durability-first
ordering by CommittedLSN, tie-break WALHeadLSN then HealthScore. All
ineligible → fail-closed, no promotion. Pluggable querier
(BlockPromotionEvidenceQuerier) for T3 wiring.
Proto messages added to volume_server.proto. gRPC transport binding
pending proto regen on M01 — this commit delivers evidence semantics
and selection substrate, not full end-to-end RPC closure.
Phase 20 doc updated with T2-T5 reviewer packs and cross-task guardrails.
13 tests: live facts, core projection mode, fail-closed no-core, 4 gated
modes, missing volume, epoch mismatch, CommittedLSN ordering, WALHeadLSN
tie-break, HealthScore tie-break, all-ineligible, mixed collection.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
