test(segmented): drive the control in a real browser

Roving focus, Home/End, wrap, toggle mode, disabled and the chosen
segment's fill are now asserted through key presses and computed styles
instead of markup greps. The static render test is deleted; its two
source sweeps move to segmented-single-implementation.test.ts.

The browser setup sets the default theme class so colour tokens
resolve, and pre-bundles the browser files' dependencies so a cold
cache does not reload the page mid-run.

src/__tests__/baselines/test-source-reads.json

@@ -1,7 +1,7 @@
 {
   "$comment": "tests that read a source file by path, per test file under src/. Maintained by src/__tests__/test-realism-ratchet.test.ts.",
-  "count": 360,
-  "total": 360,
+  "count": 359,
+  "total": 359,
   "entries": {
     "__tests__/account-payload-consumer-guard.test.ts": 1,
     "__tests__/admin-data-wipe-completeness.test.ts": 1,
@@ -264,7 +264,6 @@
     "components/ui/__tests__/card-density.test.tsx": 1,
     "components/ui/__tests__/card-header-rows.test.ts": 1,
     "components/ui/__tests__/filter-bar.test.tsx": 1,
-    "components/ui/__tests__/segmented.test.tsx": 1,
     "hooks/__tests__/use-insight-status.test.ts": 1,
     "lib/__tests__/geo-offline-detection.test.ts": 1,
     "lib/__tests__/query-keys.test.ts": 1,