chore: prepare v4.0.2 release readiness

[kevintseng]

Summary

• bump release candidate metadata to v4.0.2 because npm v4.0.1 is already published
• replace stale @xenova/transformers with maintained @huggingface/transformers and remove the ineffective consumer override
• add favicon 204 route, Level 0 ONNX capability reporting, release docs, archived plans marker, and Obsidian note sync

Verification

• npm test: 29 files, 452 tests passed
• npm run typecheck
• npm run build
• npm audit --omit=dev --json: 0 vulnerabilities
• npm run test:packaged
• clean-machine packed install smoke: install, audit, memesh --version, remember, recall
• packaged dashboard browser smoke: /dashboard rendered v4.0.2 with 0 Playwright console warnings/errors

Release Status

• npm latest remains v4.0.1 at gitHead c936c25
• v4.0.2 is prepared but not published