docs(changelog): flag NodeEvent.pre_state Any widening for observer authors

Author: chris-colinsky

CHANGELOG.md

@@ -27,6 +27,7 @@ The format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/). The
 
 - **Pinned spec version: 0.10.0 → 0.16.0.** Adopts the skip-ahead governance principle: the submodule jumps across v0.11.0–v0.16.0 (proposals 0009, 0011, 0014, 0015, 0016, 0017, 0018) in one bump. Only the surfaces introduced by proposals 0014–0017 are implemented in the batch's release; fixtures from 0011 are deferred-skip in the conformance suite and unmark with PR-5.
 - **`CheckpointRecord.schema_version` semantic shift (proposal 0014).** Previously a backend-internal record-shape version (`CHECKPOINT_SCHEMA_VERSION = "1"` constant), now the user-facing state-schema version per spec §10.2. The framework reads `type(state).schema_version` at save time. Pre-PR-4 records carrying `"1"` are reinterpreted as user-facing v1 identifiers; users with such records either declare `schema_version="1"` on their state class or discard the pre-PR-4 records. `SQLiteCheckpointer` no longer rejects records with non-default `schema_version` at the backend boundary; version-mismatch routing is now an engine concern at resume time. The `CHECKPOINT_SCHEMA_VERSION` module constant is removed; future record-shape evolution can add backend-private metadata fields if needed.
+- **`NodeEvent.pre_state` typed `Any` (was `State`).** Required by the new `checkpoint_migrated` phase which carries a `_MigrationSummary` payload rather than a `State` instance. Observer authors who type-narrowed `pre_state` to `State` should treat it as `Any` and narrow per-phase (e.g., `if event.phase == "completed": ...`). The `checkpoint_saved` phase already carried a State-flavored shape (not necessarily a typed `State` subclass instance), so this widens the declared type to match runtime reality rather than introducing a new constraint.
 
 ### Notes