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/memory-budget.ts

@@ -24,9 +24,26 @@ function readCgroupBytes(path: string): number | null {
   }
 }
 
+/** `inactive_file` from a cgroup memory.stat file — reclaimable page cache. */
+function readInactiveFile(statPath: string): number {
+  try {
+    const m = /^inactive_file (\d+)$/m.exec(fs.readFileSync(statPath, 'utf8'));
+    return m ? Number.parseInt(m[1]!, 10) : 0;
+  } catch {
+    return 0;
+  }
+}
+
 /**
  * Available headroom under the cgroup memory limit (v2 then v1), or null
  * when uncontained (no limit, non-Linux, or unreadable). Never throws.
+ *
+ * Reclaimable page cache (`inactive_file`) is credited back: `memory.current`
+ * counts it as usage, but the kernel reclaims it on demand — after a bulk
+ * parse the cache is stuffed with the DB's own pages, and the naive
+ * `max − current` read 57MB of headroom on a 6GB container and silently
+ * disabled the resolver pool (§7a.1 diagnostic run). This is the same
+ * working-set convention `docker stats` uses.
  */
 export function cgroupMemoryAvailable(): number | null {
   if (process.platform !== 'linux') return null;
@@ -34,7 +51,8 @@ export function cgroupMemoryAvailable(): number | null {
   const v2Max = readCgroupBytes('/sys/fs/cgroup/memory.max');
   if (v2Max !== null) {
     const current = readCgroupBytes('/sys/fs/cgroup/memory.current') ?? 0;
-    return Math.max(0, v2Max - current);
+    const reclaimable = readInactiveFile('/sys/fs/cgroup/memory.stat');
+    return Math.max(0, v2Max - Math.max(0, current - reclaimable));
   }
   // v1
   const v1Limit = readCgroupBytes('/sys/fs/cgroup/memory/memory.limit_in_bytes');
@@ -42,7 +60,8 @@ export function cgroupMemoryAvailable(): number | null {
   // anything at or beyond half the address-space-ish range as uncontained.
   if (v1Limit !== null && v1Limit < 2 ** 60) {
     const usage = readCgroupBytes('/sys/fs/cgroup/memory/memory.usage_in_bytes') ?? 0;
-    return Math.max(0, v1Limit - usage);
+    const reclaimable = readInactiveFile('/sys/fs/cgroup/memory/memory.stat');
+    return Math.max(0, v1Limit - Math.max(0, usage - reclaimable));
   }
   return null;
 }