fix(knowledge): the browse vocabulary catches up three kinds, and a snippet's mirror survives the generic door (#3128 recs 2-6)

Spike #3128 found the storage sound and the retrieval vocabulary frozen
before `issue` shipped (0065). Five things, in the order they had to land.

**The mirror (rec 5, the data-integrity one).** `notes.data` is DERIVED from
a snippet's body, but only `update_snippet` knew that. `update_note` is a
hasattr loop with no snippet awareness, and both doors reach it — so PATCH
/api/notes/<snippet_id> {body} rewrote the body and left the mirror behind.
`snippet_fields` PREFERS the mirror, so the row went on reporting its old
repo/path/symbol to the location reverse lookup and to prior-art recall while
displaying its new body: surfaced with full authority, and wrong.
`snippets.recompose_data` rebuilds it from the body, carrying `verification`
and `provenance` (neither is in the body to parse). An explicit `data` still
wins, so every snippet-service write is untouched.

**One facet table (rec 3), before adding any facet.** The type predicate was
written three times — SQL, Python over semantic candidates, and a ternary
computing the `is_task` pre-filter — and agreed only by luck. Adding `issue`
to the SQL arm alone would have set the pre-filter to is_task=False, handed
the Python arm a candidate set with no tasks in it, and returned an empty
semantic half for the Issues facet forever with nothing red. `_FACETS` now
generates all three. The Python arm also regains the `status IS NULL` half its
SQL twin always had.

**Issue and spike become facets (rec 2).** 435 issues — 17% of every task —
were filterable nowhere on the human surface, while retired `plan` (90 rows)
had a chip of its own. `_VALID_TYPES` was a hand-kept copy and is now derived.
`plan` stays a valid facet for its legacy rows; it loses its chip.

**Snippets stop being half-present in the feed (rec 4).** All 90 were in the
All list, in no count, wearing an empty badge, and opening in the note editor.
Counts now group by task_kind — every kind for the same two round-trips, which
is why `issue` had no number — and total includes snippets, so the All chip
matches the list it labels. Snippet cards route to /snippets/:id.

**The prose that excused it (rec 6).** `snippet_fields` and the `data` column
both still said pre-0070 rows were "never backfilled". True when 0070 landed,
false since `backfill_snippet_data` shipped, and it read as licence for a
stale mirror.

Tests: the pre-filter can never exclude a row its own facet accepts (the
regression, parameterised over every facet); both dialects select exactly
their own rows; an unknown facet matches nothing; the mirror follows a body or
title write, carries the verdict, and yields to an explicit `data`.
`compiled_sql` moves to tests/helpers rather than becoming a third copy.

Write-up: note #3161.

tests/helpers.py

@@ -12,6 +12,16 @@ from types import SimpleNamespace
 from unittest.mock import AsyncMock, MagicMock
 
 
+def compiled_sql(element) -> str:
+    """A SQLAlchemy clause or statement rendered as literal SQL text.
+
+    For asserting on the shape of a predicate without a database — which is how
+    the visibility clauses and the knowledge facets are both tested. Was a
+    private copy in each of those modules before #3128 needed a third.
+    """
+    return str(element.compile(compile_kwargs={"literal_binds": True}))
+
+
 def make_mock_session() -> AsyncMock:
     """A stand-in for ``async_session()`` — usable as ``async with``, with the
     commit/refresh/add surface a service touches.