fix: link valid stream insertion return values

[kbuffardi]

Summary

• Disable C++ exceptions for the bundled no-exception WASI libc++abi, preventing valid stream insertion from linking against unavailable __cxa_* symbols.
• Add artifact-aware Clang -> LLD -> WASI E2E coverage for defined int and std::string return values streamed through std::cout.
• Keep an undefined-function control that verifies the linker reports the user symbol, then run the compiler E2E in PR CI after fetching toolchain artifacts.
• Update the Chromium smoke program to compile and run the stream-insertion cases through the real editor flow.
• Document that try, catch, and throw are unavailable in the bundled toolchain, and bump the extension version to 0.4.6.

Verification

• npm run lint
• npm run test:e2e:compiler
• npm run build
• npm run test:e2e
• npm run test:browser:chrome (artifact-aware compiler E2E passed; the browser subprocess did not emit a completion line through this environment's runner)

Closes #74