runner: never embed a fossil screenshot from a collided run dir

!testme gitea PR #10 (drone build 35, 2026-10-05) rendered a summary card whose
app screenshot was custom-html-tiny's — a fossil from a May-31 run dir with the
same number. Root cause chain:
- /var/lib/cc-ci-runs/ keeps old-era run dirs numbered up to 1377; drone's
  build counter restarted when its DB was re-created (2026-09-27), so new
  builds collide with old dirs (35 was one).
- the run reused the collided dir without cleaning it: fresh results.json/
  summary.* landed NEXT TO the old screenshot.png.
- screenshot.capture()'s SCREENSHOT-hook branch skipped the actual snap when
  out_path already existed ('the hook may have saved it' — no hook does), so
  capture() 'succeeded' without writing and the card embedded the fossil.

Fix, cosmetics-only (R7 — verdict logic untouched):
- results.fresh_artifact_dir(): at run start, remove ONLY the files/dirs this
  run owns (results.json, summary.*, badge.svg, lint.txt, screenshot.png,
  junit/) from its artifact dir; anything else is left alone; never raises.
- run_recipe_ci calls it right after computing run_artifact_dir.
- screenshot.capture(): the hook branch ALWAYS snaps (same settle/blank-retry
  path as the default branch) — a pre-existing file is never trusted.

Unit tests: fresh_artifact_dir (owned-only removal, dir creation, odd entries)
+ capture() fake-playwright regression proving a fossil out_path is overwritten.

runner/run_recipe_ci.py

@@ -1184,6 +1184,10 @@ def main() -> int:
     # {tier,source,file,rc,junit} record; after the run we assemble results.json (per-stage/per-test +
     # level) into the artifact dir. Best-effort — never changes the verdict (R7).
     run_artifact_dir = os.path.join(results_mod.runs_dir(), results_mod.run_id())
+    # Collision guard (2026-10-05): drone build numbers restarted when its DB was re-created while
+    # old-era run dirs (same numbers, other recipes' artifacts — e.g. a fossil screenshot.png) are
+    # still on disk. Wipe the files THIS run owns so nothing stale survives into the card.
+    results_mod.fresh_artifact_dir(run_artifact_dir)
     junit_dir = os.path.join(run_artifact_dir, "junit")
     records: list[dict] = []