[AZ-447] autodev Steps 1-4 baseline: docs, tests, refactor specs

Captures the full output of autodev existing-code Phase A through
Step 4 (Code Testability Revision) for the Azaion UI workspace:

- Step 1 Document: _docs/02_document/ (FINAL_report, architecture,
  glossary, components/, modules/, diagrams/, system-flows,
  module-layout) plus _docs/00_problem/ + _docs/01_solution/ +
  _docs/legacy/ + _docs/how_to_test + README.
- Step 2 Architecture Baseline: architecture_compliance_baseline.md.
- Step 3 Test Spec: _docs/02_document/tests/ (environment,
  test-data, blackbox/performance/resilience/security/
  resource-limit tests, traceability-matrix), enum_spec_snapshot,
  expected_results/results_report.md (98 rows), plus the
  run-tests.sh + run-performance-tests.sh runners.
- Step 4 Code Testability Revision: 01-testability-refactoring/
  run dir (list-of-changes C01-C07, deferred_to_refactor,
  analysis/research_findings + refactoring_roadmap) and the 7
  child task specs AZ-448..AZ-454 under _docs/02_tasks/todo/
  plus _dependencies_table.md.
- _docs/_autodev_state.md pins the cursor at Step 4 / refactor
  Phase 4 entry so /autodev resumes cleanly.

Epic AZ-447 (UI testability gates) tracks the 7 child tasks that
will land in subsequent commits.

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

_docs/02_document/modules/src__hooks__useResizablePanel.md

@@ -0,0 +1,70 @@
+# Module: `src/hooks/useResizablePanel.ts`
+
+> **Source**: `src/hooks/useResizablePanel.ts` (33 lines)
+> **Topo batch**: B1 (leaf)
+
+## Purpose
+
+React hook that backs a draggable splitter between two horizontally-arranged panels. Owns the panel width as state and exposes a mouse handler the host attaches to the splitter element.
+
+## Public interface
+
+```ts
+export function useResizablePanel(
+  initialWidth: number,
+  min?: number,   // default 100
+  max?: number,   // default 600
+): {
+  width: number
+  onMouseDown: (e: React.MouseEvent) => void
+  setWidth: React.Dispatch<React.SetStateAction<number>>
+}
+```
+
+`width` is the current panel width (px). `onMouseDown` should be wired to the splitter element. `setWidth` is exposed for programmatic resets — currently unused by callers but kept for parity with the persistence story (see Notes).
+
+## Internal logic
+
+1. `useState(initialWidth)` for `width`.
+2. Drag bookkeeping is held in three `useRef` cells (`dragging`, `startX`, `startWidth`) — kept out of state so the move/up handlers never re-render the host.
+3. `onMouseDown` (memoized via `useCallback([width])`) snapshots `clientX` and `width`, marks `dragging = true`, and calls `e.preventDefault()` to avoid triggering text selection.
+4. A `useEffect` registers global `mousemove` / `mouseup` listeners on `window`. While `dragging.current === true`, `mousemove` updates `width` to `clamp(startWidth + (e.clientX - startX), min, max)`. `mouseup` flips `dragging` back to `false`.
+5. The effect cleans up on unmount.
+
+## Dependencies
+
+- **Internal**: none.
+- **External**: `react` (`useState`, `useCallback`, `useRef`, `useEffect`).
+
+## Consumers (intra-repo)
+
+- `src/features/annotations/AnnotationsPage.tsx` — left + right panel widths.
+- `src/features/dataset/DatasetPage.tsx` — left + right panel widths.
+
+Both pages persist their layout server-side via `UserSettings.{annotationsLeftPanelWidth, annotationsRightPanelWidth, datasetLeftPanelWidth, datasetRightPanelWidth}` (see `src/types/index.ts`). The persistence read/write happens in the page, not in this hook.
+
+## Data models
+
+None.
+
+## Configuration
+
+None.
+
+## External integrations
+
+DOM only — `window.addEventListener('mousemove' / 'mouseup')`.
+
+## Security
+
+None.
+
+## Tests
+
+None.
+
+## Notes / open questions
+
+- The hook is keyboard- and touch-blind: only mouse drag is supported, so accessibility for non-pointer input is missing. Track for the broader a11y pass; out of scope for `/document`.
+- `setWidth` is exposed but no current caller hydrates `initialWidth` from `UserSettings` via it — they pass the persisted width directly to `useResizablePanel(persistedWidth)` on mount. If the persisted width arrives **after** mount (asynchronous load), the panel jumps. Flag for the consumers' module docs (B8).
+- Default bounds (100 / 600) are arbitrary; consumer pages may want different ceilings (e.g., the annotations sidebar). Currently both consumers accept the defaults.