feat(6249): the write-path hint asks whether a new definition copies a close snippet

The derive check matches by name, so a renamed copy of a recorded snippet
passed it and showed only as "similar" prior art. When a write introduces a
definition the file does not hold (shape_ledger.new_shapes_at) and an unseen
snippet on the menu clears the write-path floor by WRITEPATH_COPY_MARGIN (0.80
at the default floor), the hint now says to reuse it or extract the shared
shape before another copy. Names the derive check or the stamping feed already
spoke for are left to them. Covers recorded snippets only: the ledger keeps no
bodies to compare an unrecorded sibling against.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/grimoire/services/shape_ledger.py

@@ -2706,6 +2706,29 @@ async def write_time_derive(
     return out
 
 
+async def new_shapes_at(
+    project_id: int, path: str, shapes: list[tuple[str, str]]
+) -> list[tuple[str, str]]:
+    """The (kind, name) shapes the hook named at ``path`` that the ledger has
+    no live row for there — definitions this write introduces, as opposed to
+    ones it edits inside. Order follows ``shapes``."""
+    wanted = [(k, n) for k, n in shapes if n]
+    if not wanted:
+        return []
+    async with async_session() as session:
+        rows = (
+            await session.execute(
+                select(CodeShape.kind, CodeShape.symbol).where(
+                    CodeShape.project_id == project_id,
+                    CodeShape.path == path,
+                    CodeShape.vanished_at.is_(None),
+                )
+            )
+        ).all()
+    here = {(k, _norm_symbol(s)) for k, s in rows}
+    return [(k, n) for k, n in wanted if (k, _norm_symbol(n)) not in here]
+
+
 async def flag_divergence(project_id: int, *, since: datetime | None) -> int:
     """Flag shapes created after ``since`` (the previous refresh) that sit
     where a canon dominates and were not proposed as that canon. With no