feat(ha-entity-snapshots): track in-flight snapshot transfer LSN per peer (task 4.1)

Adds snapshot_inflight_lsn[MAX_HA_CANDIDATES] to ha_state_t, alongside the
existing snapshot_inflight[] flag it mirrors exactly: set to the
checkpoint's LSN at ha_snapshot_send_dispatch before a transfer's first
byte goes out, cleared to 0 at every completion/failure path
(snapshot_lane_fn's normal completion, and all 3 early-return failure
sites: malloc failure, slowlane dedup-refuse, slowlane submit failure).

This is the foundation task 4.3 will consult from rotate_snapshots to
avoid deleting a checkpoint file a peer is still mid-transfer reading
from -- the one genuinely new retention guard this feature needs (the
existing write_snapshot-before-trim_wal_segments sequencing already
satisfies the WAL-trim side of Requirement 3).

Proven via a new gated unit test (test_ha_liveness.c) that observes the
tracked LSN mid-flight while the send body is parked at a test gate, not
just before/after -- confirms the lifecycle ordering, not just the
endpoints.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JDYFbrgyDHbvNYcxApKhrX

.kiro/specs/ha-entity-snapshots/tasks.md

@@ -50,7 +50,7 @@
   - _Boundary: Persistence Layer (write_snapshot)_
 
 - [ ] 4. Core: protect an in-flight checkpoint transfer from rotation
-- [ ] 4.1 (P) Track which checkpoint position each in-flight peer transfer references
+- [x] 4.1 (P) Track which checkpoint position each in-flight peer transfer references
   - Add a per-peer tracking array alongside the existing in-flight-transfer flag; set it when a
     transfer begins, clear it when the transfer completes
   - Observable: a unit test confirms the tracked position is set before a transfer's first byte
@@ -131,3 +131,4 @@
 - (2.2) First attempt ALSO touched `load_snapshot`/`persist.h` to make task 2.1's round-trip tests pass end-to-end, but review found it advanced `current_lsn`/`checkpoint_lsn` BEFORE the entity-decode loop ran -- a live silent-divergence hazard reachable through the unmodified `ha_handle_snapshot_chunk` production path (reachable today, not hypothetical, since `write_snapshot` now always emits entity-bearing checkpoints). REMEDIATED by fully reverting `load_snapshot`/`persist.h` to pre-2.2 state (byte-identical to commit `0f344da`) and rewriting the round-trip tests to parse `write_snapshot`'s raw on-disk bytes directly (new `snapent_read_checkpoint_jobs` helper using `read_Job`/`decode_Job`) instead of round-tripping through `persist_load_snapshot`. Task 5.2 must implement `load_snapshot`'s entity-decode from scratch per design.md's restore-before-advance-position contract -- do not assume any decode scaffolding exists yet.
 - (3.1) `snapent_read_checkpoint_queues`'s job-section skip loop duplicates `snapent_read_checkpoint_jobs`'s (task 2.2) skip logic verbatim rather than sharing a helper -- minor, non-blocking; consider factoring a shared "skip past job section" reader if task 5.x's test coverage grows the file further.
 - (3.2) Pre-existing ASan leak in the test fixture itself (`snapent_make` -> `makeInstance`/`makeJob`/`makeQueue`/`xkvmgr_add`, ~18KB/113 allocations), unrelated to any production code -- present since task 1.1, masked by the project's standing `ASAN_OPTIONS=detect_leaks=0` convention for this suite. Not blocking any task; worth a dedicated fixture-teardown cleanup pass if this test file keeps growing.
+- (4.1) `snapshot_inflight_lsn[peer_idx]` clears to 0 BEFORE `snapshot_inflight[peer_idx]` clears to 0 in every path (completion + all 3 failure sites in `ha_snapshot_send_dispatch`). Task 4.3's `rotate_snapshots` consultation must therefore treat `snapshot_inflight_lsn[i] != 0` as the "protected" signal directly (checkpoint LSNs are never 0), not "check flag==1 first, then trust LSN" -- there's a narrow window where the flag is still 1 but the LSN has already cleared.