test(street-level): harden KartaView tests (mutation score 51% → 95%)

A mutation review (91, then 130 mutants) found tests that could not fail
(quick presses with synchronous fakes, a no-flight check with a provider
that never posed), and untested paths. New and rewritten tests kill 123 of
130 mutants; the 7 survivors are equivalent or harmless redraws.

- viewer: held-back loads for quick presses, stale sharp/flat loads,
  close mid-load, ends checks for an older photo, WebGL retried per mount,
  focus handed back, look direction kept on ›.
- provider: id-ordered pages with out-of-order indexes, page and probe
  failures retried, overtaken lookups, cones re-centred on ‹ ›, leftover
  imagery filters hide no line or cone, coverage hints, tile cache.
- source: 429 backoff and Retry-After under mock timers, hang timeouts,
  the nearest query's parameters. policy: storage-host injection, FOV 360,
  longitudes past ±180. panorama: a stub WebGL context (angles, lost
  context, re-measuring). lineCoverage.test.mjs: tooHigh hint, passes().
- core: openImage({frame:false}) and openNearest leave the camera
  (with a posing provider and a default-open control), KartaView's legend
  and wording end to end; viewerHost supportsRenderModes.
- Fixed tick-count waits replaced by an until() poll.
- Browser gate fixture: odd photos archived (409 at storage2, served by
  the proxy), the first lookup rate-limited (429, Retry-After: 0); the
  line-click step asserts the camera stays and › skips the archived photo.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

scripts/format-scope.json

@@ -921,6 +921,7 @@
   "src/layers/streetLevel/providers/kartaview/panorama.test.mjs",
   "src/layers/streetLevel/providers/kartaview/viewer.test.mjs",
   "src/layers/streetLevel/providers/kartaview/provider.test.mjs",
+  "src/layers/streetLevel/lineCoverage.test.mjs",
   "src/testSupport/kartaViewFakes.mjs",
   "src/testSupport/googleStreetViewFakes.mjs",
   "server/providers/mapillary.js",