fix(projection): align projection with ADR 0002

Use lifetime written bytes from the current segment's newest published
sample in the headline formula, select the Percentage-Used-implied
baseline after two increments in the segment (otherwise show the
too-coarse fact), and judge horizon agreement across existing horizons.
Make the vacuous projection tests assert unconditionally and add
coverage for implied-baseline gating and single-horizon agreement.

src/fenris/projection.py

@@ -150,16 +150,40 @@ def _get_all_days(conn):
             for r in cursor.fetchall()]
 
 
+def _get_latest_published_sample(conn, segment_id):
+    """Newest published sample in the segment: (lifetime bytes written, PU).
+
+    W_t is the lifetime counter, not a regime delta.  Pending staging lives
+    in a separate table (ADR 0011), so only published samples are seen here.
+    bytes_written is DUW x 512000 as stored by the collector; fall back to
+    that conversion if only the raw counter is present.
+    """
+    if segment_id is not None:
+        cursor = conn.execute(
+            "SELECT COALESCE(bytes_written, data_units_written * 512000), percentage_used "
+            "FROM samples WHERE segment_id = ? ORDER BY id DESC LIMIT 1",
+            (segment_id,),
+        )
+    else:
+        cursor = conn.execute(
+            "SELECT COALESCE(bytes_written, data_units_written * 512000), percentage_used "
+            "FROM samples ORDER BY id DESC LIMIT 1"
+        )
+    row = cursor.fetchone()
+    return (row[0], row[1]) if row else (None, None)
+
+
 def _get_latest_pu(conn):
     cursor = conn.execute("SELECT percentage_used FROM samples ORDER BY id DESC LIMIT 1")
     row = cursor.fetchone()
     return row[0] if row else None
 
 
-def _get_pu_increments_in_segment(conn, segment_opened_at):
+def _get_pu_increments_in_segment(conn, segment_id):
     cursor = conn.execute(
-        "SELECT COUNT(DISTINCT percentage_used) FROM samples WHERE ts >= ?",
-        (segment_opened_at,),
+        "SELECT COUNT(DISTINCT percentage_used) FROM samples "
+        "WHERE segment_id = ? AND percentage_used IS NOT NULL",
+        (segment_id,),
     )
     row = cursor.fetchone()
     return max(0, (row[0] if row else 0) - 1)
@@ -186,11 +210,24 @@ def _wall_clock_in_range(conn, start, end):
 # Baseline resolution (§6.1, §6.2)
 # ---------------------------------------------------------------------------
 
+def _resolve_implied_baseline(conn, current_segment, facts):
+    """No override exists: Percentage-Used-implied tier, or unavailable (§10)."""
+    if current_segment is not None:
+        _, p = _get_latest_published_sample(conn, current_segment["id"])
+        increments = _get_pu_increments_in_segment(conn, current_segment["id"])
+        if (increments >= IMPLIED_MIN_PU_INCREMENTS and p is not None
+                and IMPLIED_P_MIN <= p <= IMPLIED_P_MAX):
+            return (BaselineTier.IMPLIED, None,
+                    "implied from vendor wear (%d%% used) — coarse" % p, facts)
+    facts.append("vendor wear estimate too coarse to imply endurance")
+    return BaselineTier.NONE, None, "no baseline", facts
+
+
 def _resolve_baseline(conn, current_segment):
     baseline = _get_baseline(conn)
     facts: list[str] = []
     if baseline is None:
-        return BaselineTier.NONE, None, "no baseline", facts
+        return _resolve_implied_baseline(conn, current_segment, facts)
 
     mandatory = [baseline["source_url"], baseline["document_revision"],
                  baseline["entry_date"], baseline["model_string"],
@@ -386,7 +423,7 @@ def _evaluate_confidence(tier, rate, regime_days, days, current_segment,
         supported_facts.append("recent data")
 
     # 5. Horizon agreement
-    if scenario_range is not None and len(scenario_range.rates) >= 2:
+    if scenario_range is not None and len(scenario_range.rates) >= 1:
         rl = list(scenario_range.rates.values())
         if min(rl) > 0 and max(rl) / min(rl) > HORIZON_AGREEMENT_FACTOR:
             failing = True
@@ -633,19 +670,17 @@ def compute_projection(conn, clock_now):
             all_facts.append(cf)
 
     headline_seconds = None
-    if state != ConfidenceState.UNSUPPORTED and rate is not None and rate > 0 and baseline is not None:
-        if tier in (BaselineTier.VERIFIED, BaselineTier.UNVERIFIED):
+    if state != ConfidenceState.UNSUPPORTED and rate is not None and rate > 0:
+        W_t, p = _get_latest_published_sample(
+            conn, current_segment["id"] if current_segment else None)
+        E_baseline = None
+        if tier in (BaselineTier.VERIFIED, BaselineTier.UNVERIFIED) and baseline is not None:
             E_baseline = baseline["tbw_terabytes"] * TBW_TO_BYTES
-        elif tier == BaselineTier.IMPLIED:
-            p = _get_latest_pu(conn)
-            if p is not None and IMPLIED_P_MIN <= p <= IMPLIED_P_MAX:
-                E_baseline = 100 * regime_bytes / p
-            else:
-                E_baseline = None
-        else:
-            E_baseline = None
-        if E_baseline is not None:
-            headline_seconds = max(E_baseline - regime_bytes, 0) / rate
+        elif (tier == BaselineTier.IMPLIED and W_t is not None and p is not None
+                and IMPLIED_P_MIN <= p <= IMPLIED_P_MAX):
+            E_baseline = 100 * W_t / p
+        if E_baseline is not None and W_t is not None:
+            headline_seconds = max(E_baseline - W_t, 0) / rate
 
     pu_line = _build_pu_context_line(conn, rate, segment_days, clock_now)