Debounced checking while typing

Nobody has claimed this yet.

## Parent

#17

## What to build

Fixing a typo clears the squiggle without saving. Checks re-run about three quarters of a second after typing stops, so the developer learns a tag is wrong before committing to it rather than after.

Blocked by the cache deliberately. Debounce without a cache means a registry request on every pause in typing, which turns a rate limit into a wall — the ordering is a correctness-of-cost decision, not an arbitrary sequence.

## Acceptance criteria

- [ ] Editing a tag re-checks shortly after typing stops, with no save required
- [ ] Typing continuously issues no requests until it pauses
- [ ] Correcting a bad tag clears its diagnostic and renders a checkmark
- [ ] Editing back and forth between two already-seen values is served from cache rather than re-requested

## Blocked by

- #27 — Persistent result cache and request concurrency cap

This issue is blocked by #27, so first read the persistent cache and request-concurrency work, then review parent issue #17 for the editing flow. Trace the existing tag check and diagnostic rendering, and verify that paused typing rechecks without saving, continuous typing sends no requests, corrections clear diagnostics, and cached values avoid repeat requests.

Written by the indexing model from the issue text.