feat(electron): add activateNow() plus fd and GC hardening

[itsjithinv]

Adds restart-free activation and fixes the descriptor/GC problems that made in-session bundle swapping unsafe.

• activateNow(): activates the staged update in the running process. Persists the incremented boot counter before returning, so crash accounting matches the restart path; rolls back instead of crash-looping when attempts are exhausted. Re-seats the boot snapshot that start() previously captured once, without which markHealthy() refuses to promote the newly booted update and the next launch rolls back a healthy release.
• bundleUrlPredicate option: lets apps that load the staged bundle themselves (e.g. loadFile to keep a file:// origin so localStorage/IndexedDB survive) keep probation crash detection working.
• Protocol handler reads files <= maxInMemoryBytes (8 MiB) instead of streaming and aborts large-file descriptors with the request, so Chromium cannot hold handles that make the bundle dir undeletable on Windows. bundleRoot may now be a function, so activation re-points it without re-registering the scheme.
• GC protects every bundle served this process, retries EACCES on deletes, and logs failures instead of swallowing them.
• Probation watchdog no longer stacks web-contents-created listeners.