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.ts

@@ -125,3 +125,40 @@ export function calibrationSamples(
   }
   return out
 }
+
+/** How the closed+estimated backlog splits into usable samples vs. what can't calibrate. */
+export interface CalibrationCoverage {
+  /** Closed issues carrying an estimate — the calibration candidates. */
+  candidates: number
+  /** Candidates that yielded a usable actual (> 0 working days) → become samples. */
+  usable: number
+  /**
+   * Candidates excluded because the issue closed with 0 working days (same-day
+   * close) or no resolvable actual — real closes that structurally can't
+   * calibrate. Counting them keeps `usable/threshold` honest: it's not "N more
+   * closes away" if some of your closes will never count.
+   */
+  excludedSameDay: number
+}
+
+/**
+ * Coverage of the calibration candidates — how many closed+estimated issues are
+ * usable vs. silently unusable (same-day / 0-day closes). {@link calibrationSamples}
+ * drops the latter; this counts them so the UI can say *why* the sample is thin.
+ */
+export function calibrationCoverage(
+  issues: GiteaIssue[],
+  timelines: Record<number, LifecycleEvent[]>,
+  asOf: Date,
+): CalibrationCoverage {
+  let candidates = 0
+  let usable = 0
+  for (const issue of issues) {
+    if (issue.state !== 'closed') continue
+    if (issue.facts.estimateDays == null) continue
+    candidates++
+    const inf = inferLifecycle(issue, timelines[issue.number] ?? [], asOf)
+    if (inf.actualWorkingDays != null && inf.actualWorkingDays > 0) usable++
+  }
+  return { candidates, usable, excludedSameDay: candidates - usable }
+}