Stream staged mount materialization

[chubes4]

Summary

• Stream staged host-to-VFS mount materialization one mount at a time instead of collecting every file payload in memory.
• Batch PHP fallback writes into bounded file payloads while preserving directory materialization and skipped directory policy.
• Add coverage that large fallback materialization splits into multiple batches.

Verification

• npx tsx tests/mount-materialization.test.ts
• npx tsx tests/staged-input-materialization.test.ts
• npm run test:recipe-runtime-setup-staged-materialization
• npm run build -- --pretty false

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Investigated the mount materialization OOM path, implemented the streaming/batching fix, added regression coverage, and ran verification commands.