feat: real Runway — per-milestone Monte Carlo forecasts

The Runway milestone list is now real. Each open gitea milestone's open scope gets
its own Monte Carlo forecast (reusing the P2 engine); the p80 landing range is
shown, and compared to the milestone's due date (on track / at risk) when one
exists. Ranges, never point dates.

- backlog.ts: runwayView(issues, milestones, deps) → RunwayMilestone[] — per
  milestone: forecast its open scope, map p50..p90 to a date range, normalize the
  RunwayBar band across a shared horizon, tone/ note from due-vs-p80. Milestones
  with no open scope (shipped) are omitted; empty → the demo fixture.
- RunwayScreen takes optional `milestones`; AppShell feeds runwayView. The header's
  calibration note was already real (#1).

Scope: each milestone forecasts its remaining work *from today* independently —
they aren't scheduled relative to each other yet (so a smaller later phase can
show an earlier date). Cross-milestone sequencing is a refinement. Capacity stays
fixture — true per-person capacity (focus factor, allocation) is #8, config-driven.

Verified: desktop typecheck clean, 14 fixture e2e green. Live: Runway shows the
real P1/P2/P4/P5 milestones with per-milestone forecasts (e.g. "P2 — Scheduler +
Monte Carlo · 80% Aug 14–25 · 32d of work"); the fixture lists Beta/Pilot/v1.0,
so the real names prove it (new assertion + screenshot).

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

apps/desktop/e2e/live-backlog.spec.ts

@@ -41,6 +41,10 @@ test.describe('live backlog', () => {
     await expect(
       win.getByText(/cold-start priors · \d+\/20 closed issues estimated|calibrated on \d+ closed/),
     ).toBeVisible()
+    // Real per-milestone forecasts — these milestone names come from gitea, not the
+    // fixture (which lists Beta / Pilot-ready / v1.0).
+    await expect(win.getByText(/P2 — Scheduler/)).toBeVisible()
+    await win.screenshot({ path: join(here, '.artifacts', 'screens', 'live-runway.png'), fullPage: true, animations: 'disabled' })
     await win.getByRole('button', { name: 'Full report' }).click()
     await expect(win.getByText(/cold-start · \d+\/20|curve active · n ≥ 20/)).toBeVisible()
     await win.screenshot({ path: join(here, '.artifacts', 'screens', 'live-calibration.png'), fullPage: true, animations: 'disabled' })