feat: add memory debug mode and settings drawer - #105

Summary

• Add an extension-wide Settings drawer with an auto-saving Memory Debug Mode toggle, defaulting to off.
• Instrument every shared compile path with fatal UBSan checks and libc++ debug hardening when enabled.
• Report supported failures with an error category and source location, preserve prior program output, and terminate nonzero.
• Link the (?) help icon to Clang's official UBSan documentation in a protected new tab.
• Prepare the updated minor release as 0.5.0.

Implementation

• Store global settings separately from project/session state in chrome.storage.local / browser.storage.local.
• Snapshot memoryDebugMode into the shared compile payload used by toolbar actions, shortcuts, and terminal g++ / clang++ commands.
• Apply -fsanitize=undefined, fatal recovery behavior, debug information, and libc++ debug hardening centrally in the compiler worker.
• Supply the missing WASI UBSan reporting runtime through explicit WebAssembly imports, decoding Clang source metadata in the worker.
• Validate the finished module against an explicit UBSan-handler allowlist so --allow-undefined cannot conceal ordinary unresolved user symbols.
• Extend Node, real-toolchain, and Chromium smoke coverage; document behavior and limitations in the README and reviewed plan.

Scope and limitations

The bundled wasm32-wasi Clang target rejects AddressSanitizer. Memory Debug Mode therefore provides the confirmed combination of UBSan and hardened libc++ checks, not comprehensive ASan heap-overflow or use-after-free detection. Generic RuntimeError: unreachable output is not used as the accepted diagnostic for supported UBSan failures.

Validation

• npm ci
• npm run lint
• npm run build
• npm run test:e2e — 216 passed
• npm run test:e2e:compiler — 7 passed against the bundled Clang/LLD/WASI toolchain
• npm run test:browser:chrome / direct Chrome smoke — Settings UI, persistence, protected help link, and compile/run fallback passed
• npm run version:check
• npm run release:check-version
• npm run package:release — produced the 0.5.0 Chromium-family archive and checksums

npm audit --audit-level=high reports the repository's existing dependency-graph findings (2 moderate, 13 high). This PR changes no dependency versions; resolving the findings—some of which require breaking build-tool upgrades—is intentionally kept out of Issue #104.

Closes #104