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/backend/src/actors/index.ts

@@ -10,9 +10,14 @@ import { resolveRunnerVersion } from "../config/runner-version.js";
 
 const runnerVersion = resolveRunnerVersion();
 
+const backendPort = process.env.HF_BACKEND_PORT ?? "7741";
+
 export const registry = setup({
   serverless: {
     basePath: "/v1/rivet",
+    configureRunnerPool: {
+      url: `http://127.0.0.1:${backendPort}/v1/rivet`,
+    },
   },
   runner: { version: runnerVersion },
   logging: {