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>

justfile

@@ -183,6 +183,15 @@ foundry-preview-logs:
 foundry-format:
 	prettier --write foundry
 
+[group('foundry')]
+foundry-dev-engine:
+	mkdir -p foundry/.foundry/logs
+	RIVET_RUN_ENGINE=1 FOUNDRY_BACKEND_PLATFORM=linux/amd64 HF_DOCKER_UID="$(id -u)" HF_DOCKER_GID="$(id -g)" docker compose --env-file .env -f foundry/compose.dev.yaml up --build --force-recreate -d
+
+[group('foundry')]
+foundry-mem-monitor interval='5':
+	./foundry/scripts/mem-monitor.sh {{interval}}
+
 [group('foundry')]
 foundry-docker-build tag='foundry:local':
 	docker build -f foundry/docker/backend.Dockerfile -t {{tag}} .