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

tests/helpers.py

@@ -191,6 +191,34 @@ def fake_snippet(**attrs) -> MagicMock:
     }, attrs)
 
 
+def fake_lesson(**attrs) -> MagicMock:
+    """A stand-in lesson: a note whose `note_type` is what makes it one.
+
+    The title carries the trigger because `compose_title` builds it that way —
+    `{what} — {when it applies}` — so a menu line rendering only the title is
+    already showing the reader when this lesson applies. Tests that used a bare
+    title here would be testing a record the product cannot create.
+
+    The check fields and `arose_from_id` are explicitly None for the reason
+    `fake_snippet`'s `data` is: `update_note` reads `verify_with` and
+    `expires_when` to decide whether to run the check-field guard, and an
+    auto-created MagicMock attribute is truthy — so a default lesson driven
+    through the update path would take a branch no real record takes.
+    """
+    attrs.setdefault(
+        "title",
+        "Give absolutely-positioned siblings an explicit stacking order — "
+        "placing two absolutely-positioned elements in the same area",
+    )
+    attrs.setdefault("data", {"when_to_apply": "two absolute siblings overlap"})
+    attrs.setdefault("status", None)
+    attrs.setdefault("arose_from_id", None)
+    attrs.setdefault("verify_with", None)
+    attrs.setdefault("expires_when", None)
+    attrs.setdefault("verified_at", None)
+    return fake_note(note_type="lesson", **attrs)
+
+
 def fake_project(**attrs) -> MagicMock:
     """design_system_id is explicit: a truthy auto-attribute would route every
     project through the design-system branch and out to a real database."""