fix(session): refuse foreign format versions before parsing current structure

Review round: the JSONL backend now refuses a foreign header version straight
from the raw header line, before validating today's header shape or decoding
any event row, so a structurally different future format reports the upgrade
direction instead of corruption (shared message builder
sessionFormatVersionRefusal). HMR live-prefix adoption runs the unknown-type
guard like the other read paths. The appendCore comment now states why the
unknown-type guard is read-side only, the loadStoredFrom JSDoc and README pin
the seek-vs-sequential refusal-scope divergence, and the generated catalog
preamble lists the ignorable envelope field.

packages/session/session-persistence/src/index.ts

@@ -38,6 +38,7 @@ export {
   PersistenceCoordinator,
   SessionFormatUnsupportedError,
   SessionPersistenceCorruptionError,
+  sessionFormatVersionRefusal,
 } from './coordinator.ts'
 export type {
   PersistenceBackend,