feat(snippets): un-merge — reverse one source out of a merged survivor

Closes the last of milestone #232. The task said to settle the design
before coding; here is what was settled and why.

THE HAZARD. Restoring a merged-in source from the trash brought the record
back but never stripped its locations off the survivor, so both claimed
the same call sites and the reverse lookup read the duplicate claims as
real. Subtracting blindly is not a fix: a location can arrive from a
source AND genuinely be the survivor's own, and _normalize_locations dedups
them into one, so blind subtraction would strip a call site the survivor
owns. Same problem defeated partial un-merge — `merged_from` recorded ids,
not which locations came from which source.

THE ANSWER. Record per-source attribution AT MERGE TIME, where it is known
exactly: each entry keeps only what that source ADDED, computed
incrementally as sources fold in. Anything the survivor already had, or an
earlier source already brought, is attributed to nobody. Both open
questions fall out of that one change — partial un-merge is exact, and a
survivor-owned location can never be stripped, because it was never
attributed in the first place.

The shape moved from [id] to [{id, locations, tags}]. Free to do: the
corpus holds one snippet and zero merges, so there is no legacy data (rule
#22). A bare int still normalizes to {"id": n} — not legacy tolerance, but
because snippet_fields falls back to PARSING THE BODY when a row has no
`data`, and the body's provenance line can only carry ids. Such an entry
shows history and refuses un-merge with a reason rather than guessing.

WHICH SURFACE. Neither option in the task, quite. Making trash-restore
notice the merge would teach the generic trash path snippet semantics for
one record type. Instead un-merge OWNS the restore: one operation, one
authorization check, trash stays ignorant. Restoring by hand is still
allowed and still leaves both records claiming the same places — so
un-merge treats an already-alive source as the normal case and goes
straight to the subtraction that repairs it. That is the state that
motivated the feature, not an error.

Adds trash.restore_entity(user_id, type, id) — the missing inverse of
delete(), which returns a batch id callers don't keep. Restores the whole
batch, since the batch is the entity plus its cascade.

Refs #2165

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

tests/test_snippet_merge_provenance.py

@@ -51,7 +51,7 @@ async def test_merge_records_what_it_absorbed_in_body_and_mirror():
         _snippet(id=1), [_snippet(id=2), _snippet(id=3)], [2, 3],
     )
     assert "**Merged from:** #2, #3" in kwargs["body"]
-    assert kwargs["data"]["merged_from"] == [2, 3]
+    assert s.merged_from_ids(kwargs["data"]["merged_from"]) == [2, 3]
 
 
 @pytest.mark.asyncio
@@ -60,7 +60,7 @@ async def test_merge_accumulates_across_merges():
     replace the record of the first."""
     target = _snippet(id=1, data=s.compose_data(name="f", merged_from=[2]))
     kwargs = await _run_merge(target, [_snippet(id=3)], [3])
-    assert kwargs["data"]["merged_from"] == [2, 3]
+    assert s.merged_from_ids(kwargs["data"]["merged_from"]) == [2, 3]
     assert "**Merged from:** #2, #3" in kwargs["body"]
 
 
@@ -74,7 +74,7 @@ async def test_merge_does_not_record_a_skipped_cross_owner_source():
         [_snippet(id=2, owner=9), _snippet(id=3, owner=42)],
         [2, 3],
     )
-    assert kwargs["data"]["merged_from"] == [2]
+    assert s.merged_from_ids(kwargs["data"]["merged_from"]) == [2]
     assert "#3" not in kwargs["body"]
 
 
@@ -93,7 +93,7 @@ async def test_an_ordinary_edit_carries_provenance_forward():
         await s.update_snippet(7, 1, signature="f(ms) -> string")
 
     kwargs = mock_update.await_args.kwargs
-    assert kwargs["data"]["merged_from"] == [2, 3]
+    assert s.merged_from_ids(kwargs["data"]["merged_from"]) == [2, 3]
     assert "**Merged from:** #2, #3" in kwargs["body"]