Build Chapter Assignment Editor add-chapter flow

Use a StreamController with debounce (from rxdart or manual Timer) for the search field to avoid filtering on every keystroke — 300ms is the standard for search fields. Compute the filtered list inside a BlocBuilder using a combination of the cubit's availableChapters list and the search query; keep the query in local widget state (StatefulWidget) since it is transient UI state, not domain state. For the 5-chapter rule: derive the effective count as (existingActiveAffiliations.length + pendingAdds.length) and show the banner when this equals or exceeds 5. The banner must use a non-dismissible design (no close button) — use a styled Container with a warning color token and an icon.

ListView.builder is mandatory given that NHF has up to 1,400 local chapters. Use an itemExtent on ListView.builder to improve scroll performance with large lists.

Write unit tests using bloc_test to verify: (1) trackPendingAdd emits the correct updated state, (2) the 5-chapter limit is enforced in the cubit (no-op or error state beyond 5). Write widget tests using flutter_test: (1) mount ChapterAssignmentEditor with a mock cubit emitting a known state, type in search field and assert list filters correctly, (2) assert already-affiliated chapters are absent from the list, (3) tap a chapter and assert trackPendingAdd is called with the correct ID, (4) simulate pending count reaching 5 and assert the MaxChaptersBanner appears, (5) assert banner disappears when count drops below 5. Test debounce behavior by advancing the fake timer 299ms and asserting no filter, then 300ms and asserting filter applied. Target 90% branch coverage.