fix(browse): headed mode on macOS 26 — stop mutating the signed Chromium bundle, heal the ones we already broke (#2242, #2138, #2139)

The in-place rebrand rewrote the Chrome-for-Testing bundle's Info.plist
(global name replace — which also renamed CFBundleExecutable to a binary
that doesn't exist) and overwrote its Resources/*.icns, breaking the
codesign seal: GPU process exit_code=5, headed mode dead on macOS 26. The
mutation lived in the SHARED Playwright cache, so it also poisoned the
user's other Playwright projects.

Three layers land together: (1) the rebrand block is gone — branding lives
in the GStack Browser.app wrapper via GSTACK_CHROMIUM_PATH, with a tombstone
and a static tripwire (no plist/icns writes into the bundle; the tripwire
allows the read-only probe below); (2) a launch-time self-heal detects an
already-poisoned cache bundle, removes it, and errors with the exact
re-fetch command — covering deploy paths that never run migrations;
(3) migration v1.64.0.0 sweeps every cached bundle, removes poisoned ones,
and re-fetches clean Chromium immediately (migrations run after ./setup, so
without the re-fetch an upgrade would end with zero working browser).
Functionally verified against fixture caches: poisoned removed, clean
untouched, rerun no-op. Migration filename tracks the final VERSION at ship.

The #2242 watchdog half is the absorbed PR #2565 (thanks @Screddyice).
Tombstone/tripwire ported from time-attack/gstack (GStack 2); self-heal and
migration are ours.

Co-authored-by: Sina Matian <sina@time-attack.dev>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

gstack-upgrade/migrations/v1.64.0.0.sh

@@ -0,0 +1,75 @@
+#!/usr/bin/env bash
+# Migration: v1.64.0.0 — repair Chrome-for-Testing bundles poisoned by the
+# old in-place rebrand (#2242).
+#
+# Why a migration: pre-v1.64 launchHeaded() rewrote the Chromium .app's
+# Info.plist ("Google Chrome for Testing" → "GStack Browser") and overwrote
+# its Resources/*.icns — inside the SHARED Playwright cache. That broke the
+# codesign seal (GPU process exit_code=5; headed mode dead on macOS 26) and
+# poisoned the cache for the user's OTHER Playwright projects too. Deleting
+# the rebrand code fixes fresh installs only; every existing macOS install
+# still has the mutated bundle on disk. This migration removes poisoned
+# bundles and re-fetches a clean one so the upgrade doesn't leave the user
+# with zero working browser (the browse launch path also self-heals, as the
+# belt to this suspenders, for installs that never run migrations).
+#
+# Affected: macOS installs that ever ran headed mode before v1.64.
+#
+# Idempotent: detection is content-based (plist contains "GStack Browser");
+# a clean cache is a no-op, and the .done touchfile gates re-runs. The
+# re-fetch is best-effort and non-fatal per the migration contract.
+
+set -u
+
+GSTACK_HOME="${GSTACK_HOME:-${HOME}/.gstack}"
+MIGRATION_DIR="${GSTACK_HOME}/.migrations"
+DONE="${MIGRATION_DIR}/v1.64.0.0.done"
+mkdir -p "${MIGRATION_DIR}" 2>/dev/null || true
+[ -f "${DONE}" ] && exit 0
+
+# macOS only: the mutation targeted .app bundle plists.
+if [ "$(uname -s 2>/dev/null)" != "Darwin" ]; then
+  touch "${DONE}"
+  exit 0
+fi
+
+PW_CACHE="${PLAYWRIGHT_BROWSERS_PATH:-${HOME}/Library/Caches/ms-playwright}"
+REMOVED=0
+
+if [ -d "${PW_CACHE}" ]; then
+  # Every Chrome-for-Testing bundle in the cache (one per pinned chromium build).
+  while IFS= read -r plist; do
+    if grep -q "GStack Browser" "${plist}" 2>/dev/null; then
+      app_dir="$(dirname "$(dirname "${plist}")")"
+      case "${app_dir}" in
+        "${PW_CACHE}"/*.app|"${PW_CACHE}"/*/*.app|"${PW_CACHE}"/*/*/*.app)
+          echo "  [v1.64.0.0] removing rebrand-poisoned bundle: ${app_dir}" >&2
+          rm -rf "${app_dir}"
+          REMOVED=1
+          ;;
+        *)
+          echo "  [v1.64.0.0] WARNING: poisoned plist outside the Playwright cache shape, skipping: ${plist}" >&2
+          ;;
+      esac
+    fi
+  done < <(find "${PW_CACHE}" -maxdepth 5 -name "Info.plist" -path "*.app/Contents/Info.plist" 2>/dev/null)
+fi
+
+if [ "${REMOVED}" = "1" ]; then
+  # Re-fetch immediately: migrations run AFTER ./setup, so without this the
+  # user finishes the upgrade with no working browser at all (headless AND
+  # headed use the same bundle). Best-effort — a failed download leaves the
+  # actionable command printed and the launch-time self-heal message covers
+  # the rest.
+  echo "  [v1.64.0.0] re-fetching a clean Chromium (bunx playwright install chromium)..." >&2
+  if command -v bunx >/dev/null 2>&1 && bunx playwright install chromium >&2; then
+    echo "  [v1.64.0.0] clean Chromium installed." >&2
+  else
+    echo "  [v1.64.0.0] WARNING: automatic re-fetch failed. Run manually: bunx playwright install chromium" >&2
+  fi
+else
+  echo "  [v1.64.0.0] no rebrand-poisoned bundles found — no-op." >&2
+fi
+
+touch "${DONE}"
+exit 0