Legacy board assignment-save guard is last-event-wins across column lanes

[Chris0Jeky]

Summary

The Legacy board carries CardModal assignment-save state from each ColumnLane through BoardCanvas into one route-level Boolean (legacyCardEditorSaving). BoardCanvas currently forwards each lane's true/false event unchanged, so the last lane to emit owns the global guard even when another lane still has an unanswered PUT.

Confirmed race

1. A card editor in column A begins an assignment save and emits true.
2. Another mounted lane emits false—for example its clean modal closes; ColumnLane.handleModalClose emits false unconditionally.
3. BoardView sets legacyCardEditorSaving = false.
4. Route leave / same-view board navigation / beforeunload is now allowed while column A's request is still in flight, despite the UI contract that submitted assignment changes cannot be discarded or cancelled.

The component graph permits one modal owner per lane, while the route guard stores only one last-event Boolean. The existing tests cover one lane only.

Expected contract

• BoardCanvas tracks assignment-save ownership independently per column lane.
• It emits the aggregate transition to true when the first lane starts saving and to false only when the final active lane settles.
• A false from one lane cannot clear another lane's owner.
• Duplicate true/false events are idempotent and do not generate misleading aggregate transitions.
• BoardView and the route-guard contract remain unchanged.

A lane disappearing while it owns an unanswered request is a distinct lifecycle boundary; do not silently weaken this fix by treating unrelated lane events as its settlement.

Acceptance

• Focused component regression with two columns: A true, B true, A false remains globally true, B false clears.
• Duplicate events remain idempotent.
• Existing Legacy assignment-save navigation tests stay green.
• Frontend lint, typecheck, build, full tests, and exact-head hosted CI pass.

Audit provenance

Found during the 2026-09-20 recent-development integrity pass while tracing async store ownership and the Legacy navigation-save protections.

[Chris0Jeky]

[Codex lane claim v2]

• lane: alpha-product-trust
• base: 307c3b8b50bec1cb0bfaea3e570a942bcb1d4451
• owned paths: frontend/taskdeck-web/src/components/board/BoardCanvas.vue plus one focused component spec
• shared-path leases: none; BoardView.vue, ColumnLane.vue, card stores and canonical docs remain untouched
• parallel-safe work: independent from test(triage): pin map-reduce and quota settlement invariants #3298, fix(board): settle deletes against current card state #3300, fix(labels): bind async commits to their board #3302 and fix(comments): order shared cache settlements #3304
• status: reproduce the two-lane last-event-wins sequence, aggregate owners in BoardCanvas, then validate the exact head