feat(desktop): add time-based staged rollout for stable updates

Linear ramp from 0% at publish to 100% at releaseDate + rolloutHours
(default 24h, configurable per release). Beta channel and rolloutHours=0
short-circuit to admit everyone. Per-machine bucket derives from a
persistent UUID at <userData>/.updaterId and feeds electron-updater's
isUserWithinRollout hook.

Adds desktop-rollout.yml workflow for in-place rolloutHours edits on
already-published releases (hotfix to 0 or extend the ramp), serialized
against finalize-rollout in desktop-release.yml via a shared concurrency
group keyed on the tag. Replaces the hand-rolled mac manifest merge
with a js-yaml round-trip that preserves unknown fields.

package.json

@@ -95,6 +95,7 @@
     "@typescript/native-preview": "7.0.0-dev.20260423.1",
     "concurrently": "^9.2.1",
     "get-port-cli": "^3.0.0",
+    "js-yaml": "^4.1.1",
     "knip": "^5.82.1",
     "lefthook": "^2.1.6",
     "oxfmt": "0.46.0",