fix: progressively scan large workspace folders

[kbuffardi]

Summary

• Replaces recursive workspace traversal with breadth-first scanning to prevent deep-folder call-stack overflows.
• Publishes a collapsed Explorer preview after each completed depth, with accessible spinners on visible folders whose subtrees are still queued.
• Preserves the previously committed workspace for compile, terminal, persistence, and file actions until the new scan completes.

Verification

• npm run lint
• npm run build
• npm run test:e2e
• npm run test:browser:chrome (Clang preflight and compiler-link tests passed; the runner did not return final browser-smoke completion status.)

Closes #88