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/harness/results.py

@@ -27,6 +27,7 @@ from __future__ import annotations
 
 import json
 import os
+import shutil
 import xml.etree.ElementTree as ET
 
 from . import level as level_mod
@@ -35,6 +36,17 @@ from . import level as level_mod
 # The dashboard serves these read-only at /runs/<run_id>/... (U0.4). Overridable for tests.
 RUNS_DIR_DEFAULT = "/var/lib/cc-ci-runs"
 
+# The artifact files/dirs a run OWNS in its run dir (written fresh every run).
+_OWNED_ARTIFACTS = (
+    "results.json",
+    "summary.html",
+    "summary.png",
+    "badge.svg",
+    "lint.txt",
+    "screenshot.png",
+    "junit",
+)
+
 
 def runs_dir() -> str:
     return os.environ.get("CCCI_RUNS_DIR", RUNS_DIR_DEFAULT)
@@ -49,6 +61,31 @@ def run_id() -> str:
     return os.environ.get("CCCI_APP_DOMAIN") or os.environ.get("CCCI_RUN_ID") or "manual"
 
 
+def fresh_artifact_dir(path: str) -> str:
+    """Ensure the run artifact dir holds no stale artifacts from a PREVIOUS run with the same id.
+
+    Drone build numbers restarted when its DB was re-created (2026-09-27) while /var/lib/cc-ci-runs
+    still holds old-era dirs numbered into the thousands — a new build reusing an old number would
+    otherwise mix old artifacts into the new run's card (observed 2026-10-05: !testme gitea build 35
+    collided with a May-31 run 35 and embedded custom-html-tiny's fossil screenshot.png, because
+    screenshot.capture() skips its snap when out_path already exists). Removes ONLY the files/dirs
+    this run owns (see _OWNED_ARTIFACTS); anything else in the dir is left untouched. Best-effort
+    (R7): never raises into the run."""
+    os.makedirs(path, exist_ok=True)
+    for name in _OWNED_ARTIFACTS:
+        p = os.path.join(path, name)
+        if os.path.isdir(p):
+            shutil.rmtree(p, ignore_errors=True)
+        else:
+            try:
+                os.remove(p)
+            except FileNotFoundError:
+                pass
+            except OSError:
+                pass  # R7: a stuck file must never fail the run — the card is best-effort anyway
+    return path
+
+
 def junit_file(junit_dir: str, tier: str, source: str, path: str) -> str:
     """Deterministic per-(tier,source,file) JUnit XML path under junit_dir."""
     base = os.path.splitext(os.path.basename(path))[0]