feat(write-path): the derive arm — the hint names a duplicate family (no canon) or a canon elsewhere for the shapes being written; exclude_derive channel (#2900, milestone 299 step 2)

shape_ledger.write_time_derive asks the ledger what it knows about each
named (kind, symbol): a derive-grouped family (identical body / same name
in N other files) -> "derive it now, do not add a copy"; a canonical row
at another path -> "canon #N at <path>, reuse". Judged rows at the path and
the canon own file stay silent. Rendered by _derive_line beside the
divergence line; keyed (group id / canon:<id>) on a third per-session dedup
channel in the hook (.derive.ids -> exclude_derive=).

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

tests/test_integration_shape_classify.py

@@ -597,6 +597,49 @@ async def test_derive_groups_land_on_rows_and_in_the_summary(seeded):
     assert {r.symbol for r in rows} == {"slug"}                   # 2 files < the name floor
 
 
+@pytest.mark.integration
+async def test_write_time_derive_names_the_family_or_the_canon_for_a_name(seeded):
+    """#2900: against real rows — a name in a dup family → the family (other
+    files, count); a name whose canonical row lives elsewhere → that canon;
+    a judged row at the path, the canon's own file, or an unknown name →
+    silence."""
+    from scribe.services.shape_ledger import apply_derive_groups, write_time_derive
+
+    owner, pid, sid = seeded["owner"], seeded["pid"], seeded["snippet"]
+    defs = _defs(
+        ("v/A.vue", "css", "log-empty", ".log-empty {", ".log-empty { color: red }"),
+        ("v/B.vue", "css", "log-empty", ".log-empty {", ".log-empty { color: red }"),
+        ("v/C.vue", "css", "log-empty", ".log-empty {", ".log-empty { color: red }"),
+        ("src/factory.py", "sym", "factory", "def factory():", "def factory():\n    return 1"),
+    )
+    await sync_repo_shapes(pid, REPO, defs, seen_marker="m1")
+    await classify_shapes(owner, pid, [
+        {"path": "src/factory.py", "symbol": "factory", "status": "canonical", "snippet_id": sid},
+    ])
+    assert await apply_derive_groups(pid) >= 3
+
+    # A 4th copy about to be written → the family, naming the other files.
+    out = await write_time_derive(pid, "v/D.vue", [("css", "log-empty"), ("css", "unknown")])
+    assert len(out) == 1 and out[0]["symbol"] == "log-empty" and out[0]["kind"] == "css"
+    fam = out[0]["family"]
+    assert fam["identical"] is True and fam["label"] == ".log-empty"
+    assert fam["files"] == ["v/A.vue", "v/B.vue", "v/C.vue"] and fam["file_count"] == 3
+    assert out[0]["key"] == fam["group"] and fam["group"].startswith("dup:")
+    # Editing one existing member still names the OTHER members.
+    out = await write_time_derive(pid, "v/A.vue", [("css", "log-empty")])
+    assert out[0]["family"]["files"] == ["v/B.vue", "v/C.vue"] and out[0]["family"]["size"] == 3
+    # The canon's name elsewhere → the canon; in the canon's own file → silence.
+    out = await write_time_derive(pid, "src/other.py", [("sym", "factory")])
+    assert out == [{"symbol": "factory", "kind": "sym", "key": f"canon:{sid}",
+                    "canon": {"snippet_id": sid, "path": "src/factory.py", "label": "factory"}}]
+    assert await write_time_derive(pid, "src/factory.py", [("sym", "factory")]) == []
+    # A judged row at the path is not re-litigated.
+    await classify_shapes(owner, pid, [
+        {"path": "v/B.vue", "symbol": "log-empty", "status": "exempt", "reason": "print sheet"},
+    ])
+    assert await write_time_derive(pid, "v/B.vue", [("css", "log-empty")]) == []
+
+
 @pytest.mark.integration
 async def test_derive_new_names_the_copy_that_joined_a_family_since_the_stamp(seeded):
     """#2899: the first sync seeds one `slug`; a later sync adds an identical