fix: fall back to Dart on native binary spawn errors; drop retired macOS x64 CI target

[ihancock]

Summary

• runNativeBinary() had no handler for spawn() launch failures. On macOS/Node 22, spawn() throws ENOEXEC synchronously for a non-executable cached binary; other failure modes (EACCES, wrong-arch) surface only via the async 'error' event. Neither path was covered, so a bad cached binary crashed the CLI instead of reaching the Dart fallback main() already implements for the "binary missing" case. Fixed by wrapping the spawn() call in try/catch and adding an 'error' listener, both routing to runWithDart().
• Root-caused the v0.9.36 release run failure: build-native-binaries (macos-13, x64) can no longer schedule because GitHub retired the macos-13 hosted runner, and current x64 macOS runner images require the paid large-runner tier. create-release requires every build-native-binaries matrix entry to succeed, so that one failure silently skipped create-release for the whole release — npm/pub.dev/vscode all published v0.9.36, but no GitHub Release or binaries were ever created, which is why the npm wrapper's download URLs 404. Dropped the macos-13/x64 matrix entry and its dependents (Homebrew SHA256 computation + on_intel block) rather than switching to a paid runner label, since that needs org-level runner/billing configuration I can't confirm is set up. npm/VSCode/IntelliJ already fall back to the Dart runtime when a native binary 404s, so Intel Mac users lose only the native-binary fast path; Homebrew on Intel now fails formula resolution with Homebrew's standard unsupported-platform error instead of trying to install a binary that no longer exists.

Test plan

• node -c bin/cli.js — syntax check passes
• Simulated a 0-byte cached binary → confirmed existing size-check fallback still reaches Dart runtime cleanly
• Simulated a non-empty but non-executable cached binary (reproduces the exact ENOEXEC synchronous-throw crash) → confirmed it now logs a warning and falls back to Dart instead of crashing
• Simulated a valid executable in the cache path → confirmed the native-binary happy path is unaffected
• python3 -c "import yaml; yaml.safe_load(...)" — release.yml parses as valid YAML after the matrix/Homebrew edits
• Full release.yml run end-to-end (requires a real v* tag push — not exercised here)

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