[AZ-455] Decompose Step 3 — test task specs (AZ-457..AZ-482)

Adds 26 blackbox-test task specs under epic AZ-455 plus the matching
rows in _dependencies_table.md. Each task depends on AZ-456 (test
infrastructure). Advances autodev existing-code flow Step 5 → Step 6
(Implement Tests, cycle 1) ready for batch implementation.

Co-authored-by: Cursor <cursoragent@cursor.com>

_docs/02_tasks/todo/AZ-468_test_header_dropdown.md

@@ -0,0 +1,50 @@
+# Test — Header Flight Dropdown a11y
+
+**Task**: AZ-468_test_header_dropdown
+**Name**: Header flight dropdown — closed/open a11y + Escape handler
+**Description**: Implement the 3 blackbox tests pinning the header flight dropdown's open/closed-state a11y attributes and the Escape-to-close handler-detachment behavior.
+**Complexity**: 2 points
+**Dependencies**: AZ-456_test_infrastructure
+**Component**: 10_app-shell (Header) (Blackbox Tests)
+**Tracker**: AZ-468
+**Epic**: AZ-455
+
+## Problem
+
+The header dropdown is keyboard-traversed dozens of times per session; an a11y regression makes the app unusable for keyboard / screen-reader users. The Escape handler must detach on close — a leak that hijacks Escape elsewhere.
+
+## Outcome
+
+- 3 scenarios pass.
+
+## Scope
+
+### Included
+
+| Scenario | Profile | Source file |
+|----------|---------|-------------|
+| FT-P-30 — header flight dropdown closed-state a11y | fast | blackbox-tests.md |
+| FT-P-31 — header flight dropdown open-state a11y | fast | blackbox-tests.md |
+| FT-N-09 — header dropdown Escape — close + handler detached | fast | blackbox-tests.md |
+
+### Excluded
+
+- Flight selection logic itself (covered in 08_test_flight_selection_persistence).
+
+## Acceptance Criteria
+
+**AC-1: Closed state**
+`aria-expanded="false"`; trigger has accessible name; no `aria-activedescendant`.
+
+**AC-2: Open state**
+`aria-expanded="true"`; `role="listbox"` (or `menu`); option list has roles; `aria-activedescendant` points to a real id.
+
+**AC-3: Escape detach**
+After Escape closes the dropdown, the document-level Escape handler installed by the dropdown is removed (tracked via `addEventListener` / `removeEventListener` spies). No leakage into other components' Escape handlers.
+
+## System Under Test Boundary
+
+- System under test: `<Header>` flight dropdown + Escape handler.
+- Allowed stubs: MSW for flights list endpoint.
+- Disallowed: reading dropdown React state.
+- Expected observables per `results_report.md` rows for FT-P-30, 31, FT-N-09.