The forge computes the "drifting pull" flow alarm: an open pull's base is 25 landings or 6 h behind main #6390

Part of #5930 (the owner's fifth alarm, 16:37Z). Rebase early, while it is cheap, before drift becomes the next conflict pile.

Done:

• [ ] src/flow/alarm_drift.ts, shaped like the other alarm_*.ts files: given the app and now, it answers { fired, since?, says }. It fires when an open pull's base commit is more than 25 landings, or 6 h, behind its repository's main.
• [ ] says names the three most-behind pulls and how far behind each is, e.g. "#4413 is 41 landings behind main; #4420 is 7 h behind."
• [ ] A test covers fired and not fired: 26 landings fires, 24 does not; a merged or closed pull never counts.

Files: src/flow/alarm_drift.ts, a test.

Before pushing: bun x tsc --noEmit and the tests you add or touch. Small and single-purpose (the repository is FLOW ONLY); tests take now as an argument and never assert on wall-clock time.