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-jsonl/src/format.ts

@@ -9,8 +9,9 @@
  */
 
 import { join } from 'node:path'
-import { decodeStorageRecord, packChunkRuns } from '@deepseek-ai/dsh-session'
+import { decodeStorageRecord, packChunkRuns, SESSION_FORMAT_VERSION } from '@deepseek-ai/dsh-session'
 import type { SessionEvent, SessionHeader, SessionId, StorageRecord } from '@deepseek-ai/dsh-session'
+import { SessionFormatUnsupportedError, sessionFormatVersionRefusal } from '@deepseek-ai/dsh-session-persistence'
 
 /** Physical encoding selected for JSONL session artifacts. */
 export type JsonlCompression = 'zstd' | 'none'
@@ -229,6 +230,22 @@ interface SessionLogScan {
 }
 
 /** Parse one complete header record supplied independently from event rows. */
+/**
+ * Refuse a header carrying a format version this build does not read BEFORE
+ * validating the current header shape or decoding any event row: a future
+ * format need not satisfy today's structural checks at all, and its user must
+ * see "upgrade the harness", never "corrupt session log".
+ * @param parsed - the JSON-parsed first line of a session artifact.
+ */
+function refuseForeignFormatVersion(parsed: unknown): void {
+  if (typeof parsed !== 'object' || parsed === null) return
+  const { version, id } = parsed as { version?: unknown; id?: unknown }
+  if (typeof version !== 'number' || version === SESSION_FORMAT_VERSION) return
+  throw new SessionFormatUnsupportedError(
+    sessionFormatVersionRefusal(typeof id === 'string' ? id : String(id), version),
+  )
+}
+
 function parseHeaderRecord(record: Buffer): SessionHeader {
   if (record.length === 0 || record.at(-1) !== 0x0A || record.indexOf(0x0A) !== record.length - 1) {
     throw new Error('empty or header-less session log')
@@ -239,6 +256,7 @@ function parseHeaderRecord(record: Buffer): SessionHeader {
   } catch {
     throw new Error('corrupt session log: header line is not valid JSON')
   }
+  refuseForeignFormatVersion(parsed)
   if (!isHeaderLine(parsed)) {
     throw new Error('corrupt session log: first line is not a session header')
   }