QVAC-2444 fix: surface worker startup stderr

[yuranich]

🎯 What problem does this PR solve?

• Native addon and Bare worker startup failures were visible only in child-process stderr.
• SDK callers saw a generic RPC_INIT_TIMEOUT error, which made failures like missing native libraries hard to diagnose from application code.

📝 How does it solve it?

• Captures a bounded tail of Bare worker stderr during Node worker startup.
• Preserves the existing RPC_INIT_TIMEOUT error class while attaching the captured worker stderr to the error cause for pre-handshake failures.
• Adds unit coverage with loadModel() and a failing worker fixture for startup failure diagnostics.

🧪 How was it tested?

• bun run build
• bun run test/unit/worker-startup-error.test.ts
• bun run test/unit/worker-close-clean.test.ts

[lauripiisang]

just one note about a possible race.

[yuranich]

/review

[github-actions]

Tier-based Approval Status

**PR Tier:** TIER1

**Current Status:** ✅ APPROVED

**Requirements:**
- 1 Team Member approval ✅ (1/1)
- 1 Team Lead OR Management approval ✅ (1/1)



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