fix(fabricator): harden framing, prevent stdio deadlocks, and add regression tests

[chrhoffmann]

Summary

This PR fixes non-deterministic hangs in bytecode fabrication by hardening parent/child framing in the fabricator path and adding focused regression tests.

Problem

pkg could hang during builds due to corrupted frame headers and stream-boundary truncation in fabricator IPC, with additional risk from debug stderr handling under backpressure.

Root Cause

• A single mutable 4-byte header buffer was reused across async stdin.write() calls.
• Child parser reset its input buffer in ways that could drop trailing bytes when chunks crossed frame boundaries.
• Debug-mode stderr wiring could contribute to pipe/backpressure deadlock patterns.

Changes

fabricator.ts

• Hardened frame parsing in the inline child script:
  • process multiple frames per chunk
  • preserve unconsumed trailing bytes between iterations
• Added defensive frame-size guards for both snap/body headers.
• Kept stderr piped and routed debug output via logger.
• Removed unexpected-close console side effects in favor of debug logging.
• Added buildFabricatorRequestChunks(...) to construct isolated frame chunks with distinct headers.
• Exported fabricatorScript for direct regression testing.

fabricator.test.ts

Added targeted unit regressions:

• header isolation (no shared header-buffer aliasing)
• multi-frame decode when payload boundaries are split across writes
• invalid size-header rejection (fast-fail path)

Validation

• npm run test:unit ✅
• npm run lint ✅
• npm run build ✅
• Regression tests reproduce the original failure modes (header aliasing, cross-boundary truncation) and pass with the fix applied.

Risk / Compatibility

• Scope is localized to fabricator framing/stdio path.
• No CLI/API surface changes.
• Low regression risk due to explicit regressions and full local checks passing.

Reviewer Checklist

• Verify frame protocol and stderr/debug handling in fabricator.ts
• Confirm regressions in fabricator.test.ts cover the reported failure modes
• Confirm CI passes on main target (yao-pkg/pkg)