feat(ui): add delay support to Spin

[wyf027]

Summary

• Add a delay prop to Spin so short loading states can avoid flashing a spinner/status region.
• Keep wrapped content aria-busy aligned with the real spinning state while delaying only the visual spinner and live region.
• Cover delayed display and cancellation in tests, and update the VitePress example with Tailwind layout classes.

Validation

• GitHub Actions CI run 28906237411 passed for commit a42a39c4450d16bb0fa0f8113be26d04bac95c69.
• CI quality job passed: Install, Lint UI, Typecheck UI, Test UI, Build UI, Build docs.
• Local npm run lint:ui, npm run typecheck:ui, npm run test:ui, and npm run build could not be executed because the local shell runner failed to create a process in this Codex environment.

[ecc-tools]

ECC bundle files are already tracked in this repository. Skipping generation of another bundle PR.