ci: anchor push routing to finished fast tier
A feat-cmux-next push now routes its Mac fast tier over the diff since the newest finished fast-tier marker on its history. A tier that reaches success or failure uploads the marker; cancelled, skipped, and reused jobs do not. Missing markers, lookup errors, and timeouts run the fast tier.
Swift test remains unchanged: workflow_dispatch and the tip Swift suite still run, while feat-cmux-next pushes still skip it. Cargo fast-tier routing is unchanged. No workflow permissions or secrets change; the lookup uses path_route's existing actions: read permission.
Request: "on feat-cmux-next pushes, cmux-next.yml's path routing must decide from the commits since the last FINISHED 'cmux-next fast tier' (success or failure; not cancelled or skipped), not since the previous push."
Design context: https://github.com/manaflow-ai/cmuxterm-hq/issues/5123#issuecomment-6515215902
Validation: all 245 targeted tests passed remotely at 42bd82a2fea583e2f9e0666f8d894975031f8da6 (11 anchor, 69 routing, 165 workflow tests). Independent Astra review returned SHIP at that exact head, checking failure marker conditions and a shallow checkout. Marker retention is one day; the new upload passes the retention guard. Routing uses the existing fast_tier output, preserving daemon-only skips from #28334.
Risk: routing may conservatively run extra Mac work when marker lookup is unavailable or history has no recorded anchor. This does not repair runner starvation or superseded-push preflight skipping.
Agent session: agent session