Stream large staged files into Playground

[chubes4]

Summary

• snapshot large staged files before VFS mutation so source/target aliasing cannot truncate the input
• stream snapshots through bounded 256 KiB PHP writes instead of one whole-file base64 allocation
• verify large writes by SHA-256 without embedding file contents in the verification payload
• preserve the existing small text/binary paths

Closes #1990.

Verification

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

All pass.

Real acceptance replay: a 65,696,427-byte FSE artifact.json that previously exhausted the 256 MB Playground PHP limit at an exact 87,595,192-byte base64 allocation now stages successfully and executes all eight SSI/WordPress/editor/visual workflow steps under the unchanged memory limit. The replay also caught and drove coverage for source/target inode aliasing, which the initial synthetic test did not model.

AI Assistance

OpenAI GPT-5.6 Terra via Homeboy/OpenCode drafted the initial bounded streaming implementation and tests. OpenAI GPT-5.6 Sol via OpenCode reviewed it, ran the real FSE replay, diagnosed source/target aliasing, and completed the snapshot-backed repair and verification. Chris Huber reviewed and is responsible for the final change.