Implement crsql_tracked_peers auto-tracking

[sinkingsugar]

Summary

The crsql_tracked_peers table has been created at extension init since forever but nothing read or wrote it. This PR wires it up so the merge path auto-records a per-peer (db_version, seq) RECEIVED watermark, and exposes crsql_set_tracked_peer for explicit application use (e.g. SENT watermarks after a successful push).

Design

• Schema: untouched. Same crsql_tracked_peers(site_id, version, seq, tag, event) table with PK (site_id, tag, event) that's been in bootstrap.c all along.
• Event constants: TRACKED_EVENT_RECEIVED = 0, TRACKED_EVENT_SENT = 1. Tag is application-defined, defaults to 0.
• Monotonic upsert: only advances when (excluded.version, excluded.seq) > (current.version, current.seq). Idempotent on equal values, safe under out-of-order arrival.
• Auto-tracking: track_peer_recv() fires once at the top of merge_insert_impl after site_id validation. Skips NULL/wrong-length site_ids and self-writes. Done eagerly so no-op merges (older CL, losing cid, idempotent re-applies) still advance the puller's watermark — otherwise we'd keep refetching the same losing changes. Best-effort: tracking errors don't fail the merge.
• Manual API: crsql_set_tracked_peer(site_id, version, seq, tag, event) mirrors the merge-path upsert so apps can record SENT watermarks (or arbitrary (tag, event) channels) without ever rolling them backwards.

Performance

Per merged row: one 16-byte memcmp, one prepared-statement reset, five binds, one step doing a single PK probe on a tiny table (one row per peer). If this ever shows up in a profile the next-level optimization is an in-ExtData accumulator flushed once per peer at commit time; the current shape is intentionally amenable to that.

Tests

• Adds testTrackedPeers covering: empty initial state; watermark written on RECEIVED merge with the correct site_id and (version, seq); idempotent re-sync (no advance); no self-row from local writes; monotonic semantics of crsql_set_tracked_peer (forwards wins, backwards no-op, equal stays put).
• All 155 py/correctness tests pass.

Side discovery (out of scope)

Existing tests in crsqlite.test.c embed sqlite3_step and sqlite3_bind_value calls inside assert(), which become no-ops under -DNDEBUG (Release). That makes a lot of the suite vacuous in Release builds — syncLeftToRight for example does literally nothing under NDEBUG. The new test uses a CHECK() macro and a local sync helper that actually exercises the code regardless of build type. Fixing the rest of the suite is worth doing, but I kept it out of this PR.

.gitignore

• .claude/ (local Claude Code state)
• npm/*/*.{dylib,so,dll} (prebuilt platform binaries staged into npm packages by the publish workflow; not source)

Test plan

• make test passes locally
• pytest (py/correctness) — 155/155 passing
• CI green on this branch