v0.40.0

No schema change: x-schema-version stays 1.19 (metadata in a set manifest
is open). A set can declare that an exercise changed its type under a kept
stable_id; two new manifest errors and a warning check the declaration,
and only a manifest that carries one can meet them. Nothing turns red on a
re-pin.

A declared type migration keeps an exercise's stable_id (engine#254)

W-CLOZE-NO-CARRIER names a better type for a cloze whose sentence is only
its blank. Following it used to cost the learner's progress: the stability
gate read the type change under the same stable_id as id reuse (V3), so
the only legal path was retire-and-mint.

• A set manifest declares the change in metadata.type_migrations
  (stable_id, from, to). Allowed transitions (TYPE_MIGRATIONS):
  cloze -> multiple_choice and cloze -> free_text.
• check-stable-ids accepts a declared exercise type change and keeps
  reporting an undeclared one as V3; it reports how many migrations the
  head declares and applies. New export stableIdDeclarations, which the CLI
  now uses for retired_ids too.
• validateManifest checks the declaration: E-TYPE-MIGRATIONS-SHAPE,
  E-TYPE-MIGRATION-PAIR, W-TYPE-MIGRATIONS-DUP.
• migrate converts a typed cloze without a carrier sentence to
  free_text, prints the type_migrations block for every converted
  exercise that keeps a stable_id, and notes a blank stable_id the new
  type cannot carry.

The consumer side was checked in the reference app before building: it keys
per-element progress on the exercise identity plus an answer-derived element
key, and the shape migrate writes keeps that key, so the progress carries
over without an app change.

Measured with this build on a copy of adaptive-learner-content (the case of
adaptive-learner-content#222): the 105 listed exercises convert (58 + 42
selects to multiple_choice, 5 typed answers to free_text), the gate
reports 105 declared and 105 applied and passes against main, the engine
gate reports 0 errors, and W-CLOZE-NO-CARRIER drops from 105 to 0.