perf: vitest threads pool + Dockerfile pnpm cache mount

Two tuning knobs that were leaving most of the host idle:

1) vitest.config.ts pool=threads with maxThreads ≈ cores/2.
   Default left this 64-core workstation at ~10% CPU during
   \`pnpm test:run\`. Threads pool uses the box: same 152-file/2050-test
   suite now runs at ~700% CPU instead of ~150%. Wall time gain is
   modest (workload is dominated by a handful of slow individual files
   that one thread must run serially), but the parallel headroom is
   there for when the suite grows. Cap = max(2, cores/2) keeps laptops
   reasonable; override with \`VITEST_MAX_THREADS=N\` in the env.

2) Dockerfile.mcpd uses BuildKit cache mounts on both pnpm install
   steps. Adds \`# syntax=docker/dockerfile:1.6\` and a
   \`--mount=type=cache,target=/root/.local/share/pnpm/store\` so
   pnpm's content-addressed store survives across image rebuilds.
   Cold rebuilds where the lockfile changed are unaffected; warm
   rebuilds where only source changed drop the install step from
   ~60s to <5s. fulldeploy.sh's mcpd image rebuild gets that back
   minus the docker push hash mismatch.

Test parity: 2050/2050 across 152 files; per-package mcpd 837/837.
Both unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

vitest.config.ts

@@ -1,8 +1,21 @@
 import { defineConfig } from 'vitest/config';
+import { availableParallelism } from 'node:os';
+
+// Default vitest's pool to ~half the CPU threads we have. The previous
+// implicit default left this 64-thread workstation at ~10% utilization
+// during `pnpm test:run`. Half is a soft cap that stays kind to laptops
+// (8-thread → 4 workers) while letting beefy hosts push closer to the
+// box's actual capacity. Override at run time with VITEST_MAX_THREADS.
+const cores = availableParallelism();
+const maxThreads = Number(process.env['VITEST_MAX_THREADS'] ?? Math.max(2, Math.floor(cores / 2)));
 
 export default defineConfig({
   test: {
     globals: true,
+    pool: 'threads',
+    poolOptions: {
+      threads: { maxThreads, minThreads: 1 },
+    },
     coverage: {
       provider: 'v8',
       reporter: ['text', 'json', 'html'],