fix(lessons): a derived mirror survives the generic note door, by kind not by name (#3734)

Groundwork for step 7, and a data-integrity fix in its own right.

Two kinds keep a queryable mirror in `notes.data` derived from their body:
snippets and, since milestone 385, lessons. Every read prefers the mirror —
deliberately, because parsing markdown to answer what an index can answer is
how a hot path rots. So a write that moves the body must move the mirror.

#3128 found that hole for snippets and plugged it with a hard-coded
`if note.note_type == SNIPPET_NOTE_TYPE`. The plug was correct and did not
generalise: lessons arrived with the same design and none of the protection,
which is precisely the "don't add a fourth instance" defect #3734 was told to
avoid.

The cost is higher for a lesson. A stale snippet mirror reports the wrong
path. A stale lesson mirror reports the wrong TRIGGER, and the trigger is the
whole retrieval story — the lesson goes on firing for the situation it used
to name while displaying the one it now names. Silent, and confident.

So `update_note` now dispatches through `_mirror_recomposers()`, a
note_type -> recomposer table. A kind with a derived mirror is covered by
registering it, not by someone remembering to widen an if.

`lessons.recompose_data` is the lesson's entry. It recovers the subject with
`embeddings.untrigger_title` — new, and deliberately placed beside the join it
inverts rather than in the caller that wanted it, because a separator spelled
in two files is a separator that will one day be changed in one of them
(#3207). `TRIGGER_SEP` is now the one spelling, and `parse_snippet_fields`
uses it too; it had the third copy inline.

The two inverses stay distinct on purpose: a snippet partitions at the first
separator (its name is a symbol), a lesson strips an exact known suffix (its
subject may legitimately contain a dash). Different algorithms, one constant,
so they cannot disagree about where the seam is.

Provenance is DROPPED when the body drops it, which is the opposite call from
a snippet's `verification` — that is carried because it was never in the body
to delete. The body is the authority; carrying a value the reader just removed
is the failure the recompose exists to prevent.

Tests: test_snippet_mirror_generic_door.py becomes
test_derived_mirror_generic_door.py, since the concern is now plural. The
registry property is asserted directly (every kind with a mirror is in the
table; the dispatch names no kind inline), plus the lesson cases and the
join/inverse round-trip. `fake_lesson` moves to tests/helpers.py — it existed
in test_lesson_surfacing.py and a second copy was about to be written — and
gains the explicit `None`s `fake_snippet` carries, because update_note reads
`verify_with` and a MagicMock is truthy.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

src/scribe/services/lessons.py

@@ -317,6 +317,46 @@ def compose_data(
     return data
 
 
+def recompose_data(note) -> dict:
+    """Rebuild a lesson's `data` mirror from its own title and body.
+
+    For the GENERIC note door. `update_lesson` composes the mirror itself from
+    the merged field set and never needs this; a plain `update_note(body=...)`
+    has no idea the mirror exists and would leave it behind.
+
+    THE COST OF LEAVING IT BEHIND IS HIGHER HERE THAN FOR A SNIPPET. A stale
+    snippet mirror reports the wrong path. A stale lesson mirror reports the
+    wrong TRIGGER — and `lesson_trigger` prefers the mirror, so the lesson goes
+    on being retrieved for the situation it used to name while displaying the
+    one it now names. The trigger is the entire retrieval story (step 3), so
+    that is not a degraded record; it is a record that fires at the wrong
+    moment and looks right when it does.
+
+    The body is the authority and the mirror is derived — already this file's
+    rule. This is its enforcement on the path that bypasses `update_lesson`.
+
+    The subject comes back out of the title through `untrigger_title`, the
+    inverse of the join that composed it, rather than by splitting on a
+    separator spelled a second time here.
+    """
+    from scribe.services.embeddings import untrigger_title
+
+    body = getattr(note, "body", None) or ""
+    trigger_match = _BODY_TRIGGER_RE.search(body)
+    trigger = trigger_match.group(1).strip() if trigger_match else ""
+    what = untrigger_title(getattr(note, "title", None), trigger)
+    # Sources through the normal read, which already falls back body →
+    # arose_from_id. A body edit that drops the provenance line should drop
+    # the mirror's copy too: the body is the authority, and carrying a value
+    # the reader just deleted is the failure this function exists to prevent.
+    sources_match = _BODY_SOURCES_RE.search(body)
+    sources = (
+        normalize_sources(_ID_RE.findall(sources_match.group(1)))
+        if sources_match else []
+    )
+    return compose_data(what, trigger, sources)
+
+
 async def create_lesson(
     user_id: int,
     *,