fix: stream large staged Playground files

[chubes4]

Summary

• stream staged files larger than 1 MiB to Playground in 256 KiB binary-safe chunks
• truncate the existing target before appending, then verify the final SHA-256 without re-embedding the large file
• retain the existing direct/text and PHP fallback paths for smaller files

Evidence

• Source relationship: fixes Stream large staged files into Playground without whole-file base64 allocation #1990 in the runtime-playground host-to-VFS staged-input materializer, the path that previously read each staged file and embedded its full base64 payload in PHP.
• Change kind: bounded-memory runtime implementation plus deterministic materialization coverage.
• Verification capability: the new test proves byte identity after truncation, binary support, exact bounded chunk payload sizes, hash-only verification, and no direct whole-file writer call for an over-threshold file.
• Scope: the runtime change is limited to files larger than 1 MiB; small-file mount semantics and established fallback behavior remain unchanged.

Verification

• npm run build
• TMPDIR=/tmp npm run test:playground-readonly-mounts
• TMPDIR=/tmp npm exec -- tsx tests/mount-materialization.test.ts
• TMPDIR=/tmp npm exec -- tsx tests/staged-input-materialization.test.ts
• TMPDIR=/tmp npm run test:playground-staged-upload-preview

AI Assistance Disclosure

OpenAI gpt-5.6-sol via OpenCode/Homeboy was used to inspect the relevant runtime path, implement the bounded-memory write flow, and add and run tests.