feat(coverage): the line names standing work with 0 unclassified + derive_new, copies that joined a family since the previous refresh (#2899, milestone 299 step 1)

Since the scoped bucket (#2869) the ledger reads 100% accounted while 439
derive rows stand; the standing block was gated on unclassified > 0 and so
went silent. Build it whatever the todo count ("; standing: ..."), and add
derive_new — derive-grouped rows first seen after the previous refresh
stamp — so entering a project names the drift ("+2 new copies since last
refresh: .error-msg in InceptionCard.vue") instead of waiting for an audit.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scribe/services/coverage.py

@@ -462,15 +462,20 @@ async def compute_coverage(
         await shape_ledger.apply_derive_groups(project_id)
     except Exception:
         logger.warning("derive-first grouping failed", exc_info=True)
-    # The button-B pass (#2793): shapes new since the PREVIOUS computation,
-    # where a canon dominates. The previous computation's stamp is the cache;
-    # a first seed has none, so it flags nothing (everything is new then).
+    # "Since the previous computation" — the cache's stamp. A first seed has
+    # none, so nothing is new then. Read once; two passes use it: the
+    # button-B flag (#2793) and the derive-new drift count (#2899).
+    since = None
     try:
         previous = await get_setting(user_id, f"{_CACHE_KEY_PREFIX}{project_id}")
-        since = None
         if previous:
             stamp = (json.loads(previous) or {}).get("computed_at")
             since = datetime.fromisoformat(stamp) if stamp else None
+    except Exception:
+        logger.warning("previous coverage stamp unreadable", exc_info=True)
+    # The button-B pass (#2793): shapes new since the PREVIOUS computation,
+    # where a canon dominates.
+    try:
         await shape_ledger.flag_divergence(project_id, since=since)
     except Exception:
         logger.warning("divergence pass failed", exc_info=True)
@@ -491,6 +496,7 @@ async def compute_coverage(
     unclassified = counts.pop("unclassified")
     proposals = shape_ledger.proposal_summary(rows)
     divergence = shape_ledger.divergence_summary(rows)
+    derive_new = shape_ledger.derive_new_summary(rows, since=since)
     return {
         "total": len(rows),
         "accounted": len(rows) - unclassified,
@@ -501,6 +507,10 @@ async def compute_coverage(
         "proposed": proposals["proposed"],
         "derive_groups": proposals["derive_groups"],
         "top_canon": proposals.get("top_canon"),
+        # Drift since the previous refresh (#2899): copies that joined a
+        # duplicate family — what the arrival line names so drift is noticed
+        # on entering, not found by an audit.
+        "derive_new": derive_new,
         "proposer": proposer_stats,
         # The divergence readout (#2793): button B where button A is canon,
         # and judged shapes whose bodies moved since they were judged.
@@ -648,29 +658,46 @@ def coverage_line(coverage: dict) -> str:
         line += f" — {breakdown}"
     line += f" (estimate{', computed ' + day if day else ''})"
     unclassified = coverage.get("unclassified", 0)
+    # The standing work, built whatever the todo count (#2899). Since the
+    # scoped bucket (#2869) a ledger can read 100% accounted and still carry
+    # derive groups, proposals and divergence; gating this block on
+    # `unclassified > 0` is how 439 derive rows went unmentioned.
+    standing = []
+    if coverage.get("proposed"):
+        standing.append(f"{coverage['proposed']} proposed")
+    n_groups = len(coverage.get("derive_groups") or [])
+    if n_groups:
+        standing.append(f"{n_groups} derive group{'s' if n_groups != 1 else ''}")
+    # Drift since the previous refresh: copies that joined a family, the
+    # first one named — the sentence the arrival moment exists to say.
+    new = coverage.get("derive_new") or {}
+    if new.get("count"):
+        n = new["count"]
+        first_new = (new.get("examples") or [{}])[0]
+        where = (
+            f": {first_new['label']} in {first_new['path']}"
+            if first_new.get("label") and first_new.get("path") else ""
+        )
+        standing.append(f"+{n} new cop{'y' if n == 1 else 'ies'} since last refresh{where}")
+    if coverage.get("divergent"):
+        standing.append(f"{coverage['divergent']} DIVERGENT")
+    # The next action, on the line (#2874): the canon with the biggest
+    # queue to confirm, and the widest body-identical copy to consolidate.
+    top = coverage.get("top_canon") or {}
+    if top.get("snippet_id"):
+        standing.append(f"top canon #{top['snippet_id']} ×{top.get('count', 0)}")
+    first = (coverage.get("derive_groups") or [{}])[0]
+    if first.get("label") and first.get("files"):
+        standing.append(f"top copy {first['label']} ×{first['files']} files")
     if unclassified:
         line += f"; {unclassified} unclassified"
-        standing = []
-        if coverage.get("proposed"):
-            standing.append(f"{coverage['proposed']} proposed")
-        n_groups = len(coverage.get("derive_groups") or [])
-        if n_groups:
-            standing.append(f"{n_groups} derive group{'s' if n_groups != 1 else ''}")
-        if coverage.get("divergent"):
-            standing.append(f"{coverage['divergent']} DIVERGENT")
-        # The next action, on the line (#2874): the canon with the biggest
-        # queue to confirm, and the widest body-identical copy to consolidate.
-        top = coverage.get("top_canon") or {}
-        if top.get("snippet_id"):
-            standing.append(f"top canon #{top['snippet_id']} ×{top.get('count', 0)}")
-        first = (coverage.get("derive_groups") or [{}])[0]
-        if first.get("label") and first.get("files"):
-            standing.append(f"top copy {first['label']} ×{first['files']} files")
         if standing:
             line += f" ({', '.join(standing)})"
         gaps = [g["dir"] for g in coverage.get("largest_gaps") or []]
         if gaps:
             line += ", largest: " + ", ".join(gaps)
+    elif standing:
+        line += f"; standing: {', '.join(standing)}"
     if coverage.get("recheck"):
         line += f"; {coverage['recheck']} judged shape{'s' if coverage['recheck'] != 1 else ''} changed since judged — recheck"
     return line

src/scribe/services/shape_ledger.py

@@ -1404,6 +1404,34 @@ def proposal_summary(rows: Iterable[CodeShape], *, top: int = 8) -> dict:
     return {"proposed": proposed, "derive_groups": ranked[:top], "top_canon": top_canon}
 
 
+def derive_new_summary(
+    rows: Iterable[CodeShape], *, since: datetime | None, top: int = 3
+) -> dict:
+    """The arrival-moment drift signal (#2899): derive-grouped rows FIRST
+    SEEN after ``since`` — the previous refresh's stamp, the same one
+    flag_divergence uses. "Since the last refresh, N more copies joined a
+    duplicate family" is the sentence that makes the derive queue a thing
+    you notice on entering, not a thing an audit finds. ``since`` None (a
+    first seed) means nothing is new. Judged rows never count."""
+    if since is None:
+        return {"count": 0, "examples": []}
+    fresh = [
+        r for r in rows
+        if r.proposal_basis == "derive" and r.proposal_group
+        and r.status in _MECHANICAL_TODO and r.vanished_at is None
+        and r.created_at is not None and r.created_at > since
+    ]
+    fresh.sort(key=lambda r: r.created_at, reverse=True)
+    return {
+        "count": len(fresh),
+        "examples": [
+            {"label": ("." if r.kind == "css" else "") + r.symbol,
+             "path": r.path, "group": r.proposal_group}
+            for r in fresh[:top]
+        ],
+    }
+
+
 async def confirm_proposals(
     user_id: int,
     project_id: int,