fix(kv/fsm): kvFSM directly satisfies raftengine.AppliedIndexReader (codex P2)

Codex round-4 P2 on kv/fsm.go:145 caught a real wiring bug: the
round-1 method was named AppliedIndexReader() returning the interface
(factory pattern), intended to be called through a separate
AppliedIndexReporter interface. But that AppliedIndexReporter was
never added to the raftengine package, and the planned Branch 3
cold-start skip gate type-asserts fsm.(raftengine.AppliedIndexReader)
directly. A factory method with a different signature does NOT
satisfy the interface, so the skip would always fall back to full
restore even after meta keys are populated on every apply.

Fix: rename kvFSM.AppliedIndexReader() to LastAppliedIndex() and
inline the type-assert forward. kvFSM now directly satisfies
raftengine.AppliedIndexReader.

Added kv/fsm_applied_index_iface_check.go with compile-time guards:

  var _ raftengine.AppliedIndexReader = (*kvFSM)(nil)
  var _ raftengine.AppliedIndexWriter = (*kvFSM)(nil)

so a future rename or signature drift fails at build time rather
than silently degrading the skip optimisation to full-restore-always.

No production callers of the old AppliedIndexReader() factory exist
(verified by grep) — the consumer is the future Branch 3 skip gate
which is not yet wired. Safe rename.

Tests: go vet + golangci-lint clean across all touched packages;
go test ./kv/ ./store/ ./internal/raftengine/... -short ok 56s.

Refs PR #915 round-4 codex P2 (kv/fsm.go:145). Codex P2 on
engine.go:4077 (snapshot artifact does not contain post-bump
metaAppliedIndex) is acknowledged but out of B2 scope — push-back
explanation in the PR comment.

Author: bootjp

kv/fsm.go

@@ -129,19 +129,36 @@ func (f *kvFSM) SetApplyIndex(idx uint64) {
 	f.pendingApplyIdx = idx
 }
 
-// AppliedIndexReader implements raftengine.AppliedIndexReporter. It
-// exposes the underlying store's durable applied-index when the
-// store implements raftengine.AppliedIndexReader (pebbleStore does;
-// the in-memory mvccStore does not, in which case (0, false, nil)
-// from the missing-key path will land at the caller). nil means
-// "not supported on this backend" and triggers the conservative
-// full-restore fallback at the cold-start skip gate. See
-// docs/design/2026_06_02_idempotent_snapshot_restore.md §3.
-func (f *kvFSM) AppliedIndexReader() raftengine.AppliedIndexReader {
-	if r, ok := f.store.(raftengine.AppliedIndexReader); ok {
-		return r
+// LastAppliedIndex implements raftengine.AppliedIndexReader by
+// forwarding to the underlying store when it supports the reader
+// seam (pebbleStore does; the in-memory mvccStore does not).
+//
+// Round-1 of this PR shipped this as a factory method
+// (AppliedIndexReader() AppliedIndexReader) intended to be called
+// through a separate AppliedIndexReporter interface. Codex P2 on
+// round-4 (kv/fsm.go:145) correctly pointed out that the planned
+// cold-start skip gate (Branch 3) will type-assert
+// fsm.(raftengine.AppliedIndexReader) directly — a factory method
+// with a different signature does NOT satisfy the interface, so
+// the skip would always fall back even after the meta key is
+// populated. Renaming the method to LastAppliedIndex and inlining
+// the type-assert forward makes kvFSM directly satisfy
+// raftengine.AppliedIndexReader.
+//
+// (0, false, nil) propagates the strictly-additive fallback when
+// the store does not expose the seam — the future skip gate treats
+// "missing" as "fall back to full restore." See
+// docs/design/2026_06_02_idempotent_snapshot_restore.md §3 / §4.
+func (f *kvFSM) LastAppliedIndex() (uint64, bool, error) {
+	r, ok := f.store.(raftengine.AppliedIndexReader)
+	if !ok {
+		return 0, false, nil
 	}
-	return nil
+	idx, present, err := r.LastAppliedIndex()
+	if err != nil {
+		return 0, false, errors.WithStack(err)
+	}
+	return idx, present, nil
 }
 
 // SetDurableAppliedIndex implements raftengine.AppliedIndexWriter by

kv/fsm_applied_index_iface_check.go

@@ -0,0 +1,9 @@
+package kv
+
+import "github.com/bootjp/elastickv/internal/raftengine"
+
+// Round-4 P2 #2 fix: kvFSM must directly implement
+// raftengine.AppliedIndexReader so the cold-start skip gate's
+// type-assertion (Branch 3) succeeds.
+var _ raftengine.AppliedIndexReader = (*kvFSM)(nil)
+var _ raftengine.AppliedIndexWriter = (*kvFSM)(nil)