fix(2): F2-3 systemic — harness.browser.goto_with_retry; applied to all install overlays

Phase 2 lesson from F2-3 (n8n install Playwright flake on net::ERR_NETWORK_CHANGED): every
install overlay that does page.goto needs the same try/except PlaywrightError + status retry.
Centralize in runner/harness/browser.py::goto_with_retry; apply to ALL install overlays.

- runner/harness/browser.py: shared helper. Polls page.goto until status in accept_statuses;
  catches PlaywrightError (net::ERR_*) as a retryable signal, not a failure. Raises AssertionError
  with last_status + last_err diagnostic only on deadline expiry.
- tests/custom-html/test_install.py: now uses goto_with_retry (200 only, wait_until=load).
- tests/custom-html/playwright/test_browser_smoke.py: same.
- tests/n8n/test_install.py: replaced inline retry loop with goto_with_retry (200, 304).
- tests/keycloak/test_install.py: goto_with_retry for admin console (200, 302, 303; 45s goto).
- tests/cryptpad/test_install.py: goto_with_retry (200, 304; 60s goto, wait_until=load).
- tests/lasuite-docs/test_install.py: goto_with_retry (200, 301, 302; 60s goto).

Cold-verifiable: ssh cc-ci 'RECIPE=custom-html cc-ci-run runner/run_recipe_ci.py'
  all 5 stages PASS (including the install overlay that flaked in the deps_smoke run),
  deploy-count=1, head_ref=8a026066==chaos-version=8a026066 (HC1 non-vacuous).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/lasuite-docs/test_install.py

@@ -10,7 +10,7 @@ import os
 import sys
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "runner"))
-from harness import generic, lifecycle  # noqa: E402
+from harness import browser as harness_browser, generic, lifecycle  # noqa: E402
 
 
 def test_serving_and_frontend(live_app, meta):
@@ -30,7 +30,10 @@ def test_serving_and_frontend(live_app, meta):
         try:
             ctx = browser.new_context(ignore_https_errors=True)
             page = ctx.new_page()
-            resp = page.goto(url, wait_until="domcontentloaded", timeout=60000)
+            # F2-3 hardening centralized in harness.browser
+            resp = harness_browser.goto_with_retry(
+                page, url, accept_statuses=(200, 301, 302), goto_timeout_ms=60_000
+            )
             assert resp is not None and resp.status in (
                 200,
                 301,