A pure read estimates when an approved pull lands, from its place in the queue and the recent landing pace #6548

Part of #5930: an approved pull says how long it will wait, so nobody pokes it.

Done:

• [ ] New src/flow/landing_eta.ts exports landingEta({ ahead, landedLastHour, medianGateMinutes }) returning minutes, or null when nothing landed in the last hour and no gate time is known.
• [ ] etaSays(minutes): "lands in about 40 min", "about 3 h", or "no estimate: nothing landed in the last hour".
• [ ] Tests: first in line, deep in line, a stalled queue, rounding.

Files: src/flow/landing_eta.ts, test/flow-landing-eta.test.ts.

Before pushing: bun x tsc --noEmit and the new test.