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/projection.py

@@ -73,6 +73,7 @@ class ScenarioRange:
     rates: Dict[int, float]
     min_days: int
     max_days: int
+    horizon_reasons: Dict[int, str] = field(default_factory=dict)
 
 
 @dataclass(frozen=True)
@@ -234,20 +235,39 @@ def _compute_regime_rate(days, conn, regime_start_day, clock_now):
     return regime_bytes / regime_wc, regime_bytes, regime_wc
 
 
-def _compute_horizon_rate(days, conn, horizon_days, clock_now):
-    cutoff = (clock_now - timedelta(days=horizon_days)).strftime("%Y-%m-%d")
+def _compute_horizon_rate(days, conn, horizon_days, evidence_endpoint):
+    """Compute horizon rate anchored at the latest evidence endpoint T.
+
+    Uses exact trailing horizon_days × 86400 seconds from T, not clock_now.
+    Reader refresh alone never moves T or dilutes rates.
+
+    Returns (rate, reason) where reason is None on success or a string
+    describing why the rate is unavailable.
+    """
+    if not days:
+        return None, "no observation history"
+
+    # T is the latest evidence endpoint — the end of the last day aggregate
+    T = datetime.fromisoformat(days[-1]["day"] + "T23:59:59+00:00")
+
+    # Exact trailing start: T minus horizon_days × 86400 seconds
+    h_start = T - timedelta(days=horizon_days)
+    cutoff = h_start.strftime("%Y-%m-%d")
+
     # History must span the full horizon — no placeholders
-    if not days or days[0]["day"] > cutoff:
-        return None
+    if days[0]["day"] > cutoff:
+        return None, "%d-day window starts before earliest data" % horizon_days
+
     h_bytes = sum(d["bytes_written"] for d in days if d["day"] >= cutoff)
     covered = sum(1 for d in days if d["day"] >= cutoff)
     if covered == 0:
-        return None
-    h_start = datetime.fromisoformat(cutoff + "T00:00:00+00:00")
-    h_wc = _wall_clock_in_range(conn, h_start, clock_now)
+        return None, "%d-day window has no data" % horizon_days
+
+    h_wc = _wall_clock_in_range(conn, h_start, T)
     if h_wc <= 0:
-        return None
-    return h_bytes / h_wc
+        return None, "%d-day window has no monitored wall-clock time" % horizon_days
+
+    return h_bytes / h_wc, None
 
 
 # ---------------------------------------------------------------------------
@@ -508,12 +528,20 @@ def compute_projection(conn, clock_now):
 
     scenario = None
     horizon_rates = {}
+    horizon_reasons = {}
     for h in HORIZON_DAYS:
-        hr = _compute_horizon_rate(all_days, conn, h, clock_now)
+        hr, reason = _compute_horizon_rate(all_days, conn, h, clock_now)
         if hr is not None:
             horizon_rates[h] = hr
+        else:
+            horizon_reasons[h] = reason
     if horizon_rates:
-        scenario = ScenarioRange(rates=horizon_rates, min_days=min(horizon_rates), max_days=max(horizon_rates))
+        scenario = ScenarioRange(
+            rates=horizon_rates,
+            min_days=min(horizon_rates),
+            max_days=max(horizon_rates),
+            horizon_reasons=horizon_reasons,
+        )
 
     total_days_count = len(segment_days)
     days_below_coverage = sum(1 for d in segment_days