[AZ-485] Add Public API barrels + STC-ARCH-01 (F4 close)

Closes architecture baseline finding F4. Every component now exposes
its Public API through `src/<component>/index.ts`; cross-component
imports go through the barrel. `scripts/check-arch-imports.mjs` plus
`STC-ARCH-01` in the static profile enforce the rule; tests in
`tests/architecture_imports.test.ts` cover AC-4/AC-5 + 2 exemption
cases. One F3-pending exemption (`classColors`) is documented in 5
places (barrel, consumer, script, doc, test) to avoid a circular
import.

Phase B cycle 1 batch 1 of 2 (epic AZ-447). Batch 2 is AZ-486
(endpoint builders) — blocked on this commit landing.

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

tests/tile_split_zoom.test.tsx

@@ -9,8 +9,8 @@ import {
   waitFor,
 } from './helpers/render'
 import { seedBearer, clearBearer } from './helpers/auth'
-import { FlightProvider } from '../src/components/FlightContext'
-import DatasetPage from '../src/features/dataset/DatasetPage'
+import { FlightProvider } from '../src/components'
+import { DatasetPage } from '../src/features/dataset'
 import {
   AnnotationSource,
   AnnotationStatus,