feat(foundry): memory investigation tooling and VFS pool spec

Add memory monitoring instrumentation, investigation findings, and
SQLite VFS pool design spec for addressing WASM SQLite memory spikes.

- Add /debug/memory endpoint and periodic memory logging (dev only)
- Add mem-monitor.sh script for continuous memory profiling with
  automatic heap snapshot capture on spike detection
- Add configureRunnerPool to registry setup for engine driver support
- Document memory investigation findings (per-actor cost, spike behavior)
- Write SQLite VFS pool spec for bin-packing actors onto shared WASM instances
- Add foundry-mem-monitor and foundry-dev-engine justfile recipes
- Add compose.dev.yaml engine driver and platform support

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

foundry/packages/client/package.json

@@ -16,7 +16,7 @@
   "dependencies": {
     "@sandbox-agent/foundry-shared": "workspace:*",
     "react": "^19.1.1",
-    "rivetkit": "2.1.6",
+    "rivetkit": "https://pkg.pr.new/rivet-dev/rivet/rivetkit@791500a",
     "sandbox-agent": "workspace:*"
   },
   "devDependencies": {