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/index.ts

@@ -73,6 +73,7 @@ export type {
 
 export {
   CALIBRATION_BUCKET_FLOOR,
+  calibrationCoverage,
   calibrationSamples,
   COLD_START_THRESHOLD,
   fitCalibration,
@@ -80,6 +81,7 @@ export {
 } from './calibration/calibration-v0.js'
 export type {
   BucketFit,
+  CalibrationCoverage,
   CalibrationModel,
   CalibrationSample,
   PersonBias,