Add regression test for Board's titleEditableViaCell primary-field/groupBy collision fallback

[brylie]

Board's titleEditableViaCell derivation (BoardCollectionView.svelte:164-171) falls back to a plain, non-editable card title whenever the Collection's primary field is also groupBy or swimlaneBy — avoiding a duplicate editable control for the same value (documented in docs/specifications/collection-views.md §7, issue #104). This behavior is currently correct only by code inspection: BoardCollectionView.svelte.test.ts has no test exercising it.

Surfaced during PR reflection on #268, which added the equivalent test for Calendar's mirror of this same pattern (CalendarCollectionView.svelte.test.ts, "falls back to a plain non-editable title when the primary field is also the groupBy date property").

• Add a Board component test setting a Collection's primaryFieldKey equal to config.groupBy, asserting the card renders exactly one editable control for that field's value (e.g. via getAllByDisplayValue length 1) and the static fallback label, not a duplicate PropertyValueCell
• Add the equivalent test for primaryFieldKey equal to config.swimlaneBy
• No production code change expected — this is a coverage gap, not a behavior change

Done when: a future refactor of Board's grouping/primary-field derivation that reintroduces a duplicate editable control for a colliding field would fail a test, not just a manual reading of the code.

Priority: P2 — matches comparable test-coverage-backfill issues on this board (#233, #221), no user-facing bug today. Targeted at 0.4.0 since it backfills coverage for #96/#104/#105, all shipped in this milestone, and needs QA closure before the release rather than drifting into 0.5.0.