test: activate the full e2e (browser-driving) suite + add fetch --force

The 138 @pytest.mark.e2e tests were doubly inactive: deselected by addopts AND
skipped without a cached binary — and 3 of the 6 per-file firefox_binary
fixtures silently ignored INVPW_BINARY_PATH, so they'd test whatever was cached
even when you pointed the suite elsewhere (a false-confidence trap).

- Centralize firefox_binary into conftest.py (env INVPW_BINARY_PATH → cache →
  skip); delete the 6 duplicates. Unify test_webrtc_realness onto the same env.
- scripts/run_e2e.py: one command that runs ALL e2e against a chosen binary,
  with reruns so an under-load interaction flake (dblclick/hover pass 3/3 in
  isolation) self-heals while a real break fails every attempt. The webrtc e2e
  fake a TCP-only SOCKS locally, so the suite is offline. This is the MANDATORY
  pre-release browser gate (local — hosted runners are too interaction-flaky).
- Running the suite against firefox-9 surfaced a real gap: `invisible_playwright
  fetch --force` was unrecognized (the subparser took no args) though the e2e
  test + docstring expect it. Implement it: drop the cached version dir, refetch.
- Add pytest-rerunfailures + playwright to the dev extras.

Baseline against firefox-9: 136 passed, 1 skipped (linux_only on win host),
1 was the --force gap now fixed.

src/invisible_playwright/cli.py

@@ -10,7 +10,15 @@ from .constants import BINARY_VERSION, FIREFOX_UPSTREAM_VERSION
 from .download import cache_root, ensure_binary
 
 
-def _cmd_fetch(_args: argparse.Namespace) -> int:
+def _cmd_fetch(args: argparse.Namespace) -> int:
+    # --force: re-download even if already cached (drop the cached version dir,
+    # then let ensure_binary fetch it fresh). Useful to recover a corrupted cache
+    # or re-pull after a re-published release.
+    if getattr(args, "force", False):
+        from .download import cache_dir_for_version
+        d = cache_dir_for_version()
+        if d.exists():
+            shutil.rmtree(d, ignore_errors=True)
     path = ensure_binary()
     print(path)
     return 0
@@ -52,7 +60,9 @@ def build_parser() -> argparse.ArgumentParser:
     )
     sub = p.add_subparsers(dest="cmd")
 
-    sub.add_parser("fetch", help="download the patched Firefox binary")
+    fetch_p = sub.add_parser("fetch", help="download the patched Firefox binary")
+    fetch_p.add_argument("--force", action="store_true",
+                         help="re-download even if already cached")
     sub.add_parser("path", help="print the absolute path to the cached binary")
     sub.add_parser("version", help="print wrapper and binary versions")
     sub.add_parser("clear-cache", help="remove all cached binaries")