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.

scripts/stamp-rollout.mjs

@@ -0,0 +1,46 @@
+import fs from "node:fs";
+import { dump, load } from "js-yaml";
+
+export function stampRollout({ releaseDate, rolloutHours }, paths) {
+  if (releaseDate === undefined && rolloutHours === undefined) {
+    throw new Error("stampRollout requires at least one of releaseDate or rolloutHours");
+  }
+  for (const filePath of paths) {
+    const manifest = load(fs.readFileSync(filePath, "utf8")) ?? {};
+    const next = { ...manifest };
+    if (releaseDate !== undefined) next.releaseDate = releaseDate;
+    if (rolloutHours !== undefined) next.rolloutHours = rolloutHours;
+    fs.writeFileSync(filePath, dump(next, { lineWidth: -1, noRefs: true }));
+  }
+}
+
+function parseArgs(argv) {
+  const opts = {};
+  const paths = [];
+  for (let i = 0; i < argv.length; i++) {
+    const arg = argv[i];
+    if (arg === "--release-date") {
+      opts.releaseDate = argv[++i];
+    } else if (arg === "--rollout-hours") {
+      const raw = argv[++i];
+      const parsed = Number(raw);
+      if (!Number.isFinite(parsed) || parsed < 0) {
+        throw new Error(`--rollout-hours must be a non-negative number, got ${raw}`);
+      }
+      opts.rolloutHours = parsed;
+    } else {
+      paths.push(arg);
+    }
+  }
+  return { opts, paths };
+}
+
+if (import.meta.url === `file://${process.argv[1]}`) {
+  const { opts, paths } = parseArgs(process.argv.slice(2));
+  if (paths.length === 0 || (opts.releaseDate === undefined && opts.rolloutHours === undefined)) {
+    throw new Error(
+      "Usage: node scripts/stamp-rollout.mjs [--release-date <iso>] [--rollout-hours <n>] <yaml-path>...",
+    );
+  }
+  stampRollout(opts, paths);
+}