calibration: count same-day closes honestly (#34)

The cold-start surface showed "N/20 closed issues estimated", implying you're
just (20−N) closes away. But calibrationSamples silently drops closed+estimated
issues that closed in 0 working days (same-day closes) — real closes that
structurally can't calibrate. On this repo that's 10 of 24 closes hidden: the
note read 14/20 as if 6 away, when a third of the history will never count.

- core: `calibrationCoverage(issues, timelines, asOf)` → { candidates, usable,
  excludedSameDay }, counting the silently-excluded same-day closes. Pure, tested.
- surface it: CalibrationData gains `excludedSameDay`; backlogCalibration returns
  the coverage; the Runway note and the Calibration screen now say "… · N same-day
  closes can't calibrate" so the thin sample is explained, not just reported.

Verified on christian/commitea: closed=24, usable=14, excludedSameDay=10.
131 core green (incl. new coverage test); core + desktop typecheck; 14 fixture e2e.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/core/src/calibration/calibration-v0.test.ts

@@ -5,6 +5,7 @@ import type { LifecycleEvent } from '../lifecycle/lifecycle-v0.js'
 import type { GiteaIssue } from '../gitea/types.js'
 import {
   CALIBRATION_BUCKET_FLOOR,
+  calibrationCoverage,
   calibrationSamples,
   type CalibrationSample,
   COLD_START_THRESHOLD,
@@ -118,4 +119,26 @@ describe('calibrationSamples', () => {
     const noEst = issue({ number: 9, labels: [] })
     expect(calibrationSamples([open, noEst], { ...events(8), ...events(9) }, asOf)).toEqual([])
   })
+
+  it('coverage counts same-day closes as excluded candidates, not as "more closes needed"', () => {
+    // usable: commit Wed 01-07 → close Mon 01-12 = 3 working days
+    const usable = issue({ number: 7, labels: ['est/2d'] })
+    // same-day close: commit and close on the same day = 0 working days → excluded
+    const sameDay = issue({ number: 10, labels: ['est/2d'], createdAt: '2026-01-12T08:00:00Z' })
+    const sameDayEvents = {
+      10: [
+        { type: 'commit', at: '2026-01-12T09:00:00Z' } as LifecycleEvent,
+        { type: 'close', at: '2026-01-12T17:00:00Z' } as LifecycleEvent,
+      ],
+    }
+    const open = issue({ number: 8, state: 'open', labels: ['est/2d'], closedAt: null })
+    const noEst = issue({ number: 9, labels: [] })
+
+    const cov = calibrationCoverage([usable, sameDay, open, noEst], { ...events(7), ...sameDayEvents }, asOf)
+    expect(cov.candidates).toBe(2) // closed + estimated only (usable + sameDay)
+    expect(cov.usable).toBe(1)
+    expect(cov.excludedSameDay).toBe(1)
+    // the honest denominator: usable matches the model's n
+    expect(cov.usable).toBe(calibrationSamples([usable, sameDay, open, noEst], { ...events(7), ...sameDayEvents }, asOf).length)
+  })
 })