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.

tests/test_cross_origin_iframe.py

@@ -31,7 +31,6 @@ random free ports.
 from __future__ import annotations
 
 import socket
-import sys
 import threading
 from http.server import BaseHTTPRequestHandler, HTTPServer
 
@@ -165,22 +164,6 @@ def cross_origin_harness():
         sb.shutdown()
 
 
-@pytest.fixture(scope="session")
-def firefox_binary():
-    """Locate the cached patched Firefox binary or skip."""
-    from invisible_playwright.constants import BINARY_ENTRY_REL
-    if sys.platform not in BINARY_ENTRY_REL:
-        pytest.skip(f"unsupported platform: {sys.platform}")
-    from invisible_playwright.download import cache_dir_for_version
-    entry = cache_dir_for_version() / BINARY_ENTRY_REL[sys.platform]
-    if not entry.exists():
-        pytest.skip(
-            "patched Firefox binary not cached; run `invisible-playwright fetch` "
-            "to enable E2E tests"
-        )
-    return str(entry)
-
-
 @pytest.mark.e2e
 def test_cross_origin_iframe_url_appears_in_page_frames(firefox_binary, cross_origin_harness):
     """``page.frames`` must list the cross-origin iframe with its real URL.