Fix staged Playground file durability

[chubes4]

Summary

• stream staged files from a private snapshot so writes to a NodeFS-mounted target cannot truncate the source being read
• verify the final mounted target byte count and SHA-256 after all chunks are written
• cover >256 KiB text, binary, and truncated-target diagnostics

Fixes #2240.

Verification

• staged file materialization preserves large text and binary files

wp-codebox-workspace@0.19.0 build
node ./node_modules/typescript/bin/tsc -b packages/runtime-core packages/runtime-playground packages/cli && node scripts/ensure-cli-bin-executable.mjs

wp-codebox-workspace@0.19.0 test:playground-staged-upload-preview
tsx tests/playground-staged-upload-preview.integration.test.ts

wp-codebox-workspace@0.19.0 test:playground-readonly-mounts-integration
tsx tests/playground-readonly-mounts-integration.test.ts

playground readonly mount integration ok

• Direct candidate CLI fixture 20 dependency discovery succeeded; the 816,891-byte generated artifact produced the WooCommerce dependency plan.

AI assistance

OpenAI gpt-5.6-terra via OpenCode diagnosed the mounted-source truncation, implemented snapshot and integrity verification, and ran focused, integration, build, and canonical dependency-discovery checks. Chris Huber reviewed and remains responsible for every line.