…310)

Fixes the intermittent `SQRLTerminationListenerSpec` failures the matrix
CI surfaced (#305, #306).

`launchApplicationAtURL:` returns immediately while the spawned process
is still doing its `NSApplication` startup. `-[SQRLTerminationListener
waitForTermination]` then calls
`runningApplicationsWithBundleIdentifier:` — if the app hasn't checked
in with Launch Services yet, that returns an empty array and the signal
completes immediately.

Both observed failure modes are this same window:
- `observedApp == nil` + `completed == YES` from the start (one-instance
test)
- `completed` going true after only one of two terminates
(multi-instance — listener only saw one app at subscribe time)

Block in the fixture until `app.finishedLaunching` so every caller
(these two specs plus the seven `SKIP_IF_RUNNING_ON_TRAVIS`-tagged
`SQRLUpdaterSpec` cases) sees a stable starting state.

Upstreams the remaining `electron/patches/squirrel.mac/` patches into
this repo so Electron can eventually drop them. Six commits, **8 files,
+537/−51**, **63 → 74 tests**.

| Commit | Upstreams | Tests added |
|---|---|---|
| `feat: SquirrelMacEnableDirectContentsWrite` |
`feat_add_new_squirrel_mac_bundle_installation_method_behind_flag` |
parent-dir-untouched e2e |
| `fix: abort install if app running; resolve target path once` |
`fix_abort_installation_attempt_at_the_final_mile_if_the_app_is` +
`fix_resolve_target_bundle_path_once_at_start_of_install` |
symlink-target rejected, abort-if-running |
| `feat: ElectronSquirrelPreventDowngrades` |
`feat_add_ability_to_prevent_version_downgrades` |
`+isVersionAllowedForUpdate:from:` units |
| `refactor: harden ShipIt launch` |
`refactor_use_posix_spawn_instead_of_nstask…` +
`fix_trigger_shipit_mach_service_after_smjobsubmit…` +
`chore_turn_off_launchapplicationaturl_deprecation…` | (existing
remote-ShipIt spec exercises new path) |
| `refactor: non-deprecated NSKeyedArchiver` |
`refactor_use_non-deprecated_nskeyedarchiver_apis` |
`SQRLInstallerOwnedBundle` round-trip |
| `fix: prune orphaned staged updates` |
`fix_clean_up_orphaned_staged_updates_before_downloading_new_update` |
unit prune + e2e bounded-count; replaces `xit` |

Fixes #124. Fixes #196. Fixes #264.

> [!NOTE]
> Two adaptations should be backported to Electron's patches:
> - `SQRLUpdater.m`: `BOOL launchPrivileged = !targetWritable` in the
original `direct-contents-write` patch shadowed the outer var — it was a
dead store
> - `SQRLInstaller.m`: `runningApplicationsWithBundleIdentifier:` throws
on `nil`; guarded it

Replaces the stacked #303 / #304 / #305 / #306 / #307 / #309. #302
(strict codesign validation) stays separate — it touches only
`SQRLCodeSignature.m`.