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_webrtc_realness.py

@@ -342,7 +342,9 @@ _FAKE_EGRESS = "203.0.113.7"  # RFC 5737 TEST-NET-3
 
 
 def _e2e_binary():
-    cand = os.environ.get("STEALTHFOX_E2E_BINARY")
+    # Honor both env vars so the whole e2e suite targets one binary from a single
+    # setting (INVPW_BINARY_PATH is what conftest's firefox_binary uses).
+    cand = os.environ.get("STEALTHFOX_E2E_BINARY") or os.environ.get("INVPW_BINARY_PATH")
     if cand and os.path.exists(cand):
         return cand
     built = r"C:\ff\source\obj-x86_64-pc-windows-msvc\dist\bin\firefox.exe"