feat(#76): anchor scenario windows at evidence endpoint T

Headline and scenario rates now describe exact evidence-supported
monitored spans anchored at the latest published usage-evidence
endpoint T, not clock_now. Reader refresh alone never moves T or
dilutes rates.

- Add horizon_reasons field to ScenarioRange for specific unavailability facts
- Modify _compute_horizon_rate to use exact trailing 7/28/90×86400-second starts from T
- Show specific reasons for affected horizons (e.g., "starts before earliest data")
- Update TUI and CLI to display horizon-specific reasons
- Add 6 new tests for evidence-anchored projection rates

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>

src/fenris/tui.py

@@ -1205,13 +1205,17 @@ class FenrisTuiApp(App):
         )
 
     def _format_scenario(self, proj: ProjectionResult) -> str:
-        """Format scenario range (spec §6.5)."""
-        if not proj.scenario_range or not proj.scenario_range.rates:
+        """Format scenario range with horizon reasons (spec §6.5)."""
+        if not proj.scenario_range:
             return ""
         parts = []
         for horizon in sorted(proj.scenario_range.rates.keys()):
             rate_gb_day = proj.scenario_range.rates[horizon] * 86400 / 1e9
             parts.append("%dd: %.2f GB/day" % (horizon, rate_gb_day))
+        for horizon, reason in sorted(proj.scenario_range.horizon_reasons.items()):
+            parts.append("%dd: %s" % (horizon, reason))
+        if not parts:
+            return ""
         return "[bold]Scenario range[/bold] · %s" % " · ".join(parts)
 
     # --- Actions ---