fix compiler failure diagnostics

[arthtyagi]

What changed

• retain the exact compiler command and numeric child exit code on CcCompileError
• preserve actual stderr instead of replacing an empty stream
• surface captured stdout when a compiler writes diagnostics there
• include a process-error fallback when both output streams are empty
• add regression coverage with a deterministic compiler wrapper

Why

The final compiler-driver wrapper used err.stderr ?? String(err). An empty stderr string therefore suppressed every other useful field from Node's child-process error, leaving cross-target failures without the command, exit code, or any fallback diagnostic.

This is intentionally limited to diagnostics. It does not add or change target support.

References #156.

Validation

• pnpm exec eslint packages/compiler/src/backend/cc.ts packages/compiler/test/cc-driver.test.ts
• compiler TypeScript build under Node 24.15.0
• packages/compiler/test/cc-driver.test.ts under Node 24.15.0, plain: 9 passed, 9 skipped
• same focused suite with SCRIPTC_SAN=1: 9 passed, 9 skipped
• regression observed failing before the production change and passing afterward

The repository-wide local fallback was attempted but could not complete in this checkout because CMake is not installed; dynamic-engine suites fail at their existing CMake prerequisite.

[vercel]

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