fix(db,resolution): WAL file cap + cgroup cache credit + pool/parse sizing corrections from the instrumented kernel-scale runs (#1335)

Four §7a.1 instrumented-run findings, each measured:

1. File-size trigger + truncate-at-barrier: a fully-backfilled WAL still
   grows the FILE without bound — the writer only restarts at frame 0 when a
   commit finds zero reader marks, which the instrumented run showed never
   happens (file marched 361→721MB through two COMPLETE backfills; 22GB by
   phase end). backpressure() now also trips at 4× the soft cap on raw file
   size and TRUNCATEs at the parked barrier; the timer path truncates
   opportunistically after complete backfills. Dubbo peak: 251MB → 69MB at
   the same 16MB valve; dumps byte-identical under aggressive folding.

2. cgroup memory credit: memory.current counts reclaimable page cache — a
   post-parse container read 57MB of headroom on a 6GB box and silently
   disabled the pool. inactive_file is credited back (the docker-stats
   working-set convention); the same run now reads a sane 4.4GB budget.

3. Pool at 2 cores reversed: sequential resolution measured FASTER than
   pooled-6-on-2 at kernel scale (853s vs 1,150s), and synthesis is
   Amdahl-bound by cFnPtrEdges (306s of 358s) so pooling it bought nothing.
   cpuCap = min(ap−1, 6), no floor: ap=2 → sequential is the fast path.

4. Parse floor of 2: one parse worker at a 2-cpuset measured 34% slower
   (493s vs 369s) — main + store-worker don't fill the second core. Floor
   restores the baseline (373.5s measured).

Plus the observability §7a.1 burned three 25-minute cycles for: valve
armed/fire/timer-pass/heartbeat lines, checkpoint-worker error capture,
pool sizing decisions (incl. the disabled path), backpressure-hook
presence — all behind CODEGRAPH_SYNTH_TIMINGS / CODEGRAPH_WAL_VALVE_DEBUG.

Suite: 2,490 passed / 4 skipped (kernel required). Kernel-scale record
runs with this build follow in the migration plan §7a.1.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/resolution/index.ts

@@ -1358,6 +1358,10 @@ export class ReferenceResolver {
     // window to fire; see ./cooperative-yield.
     const maybeYield = createYielder();
 
+    if (process.env.CODEGRAPH_SYNTH_TIMINGS) {
+      console.error(`[pool-timing] backpressure hook: ${parallel?.backpressure ? 'present' : 'absent'}`);
+    }
+
     await this.warmCachesYielding(maybeYield);
 
     const total = this.queries.getUnresolvedReferencesCount();