fix(session-title): typed rename rejection, provenance invariant, contract docs

SessionTitleInvalidError narrows the one rename failure that blames the
input; the fallback-unpin append extracts to appendFallback beside
ensureFallback's guarded twin; a deferred-provider test proves rename
supersedes ACTIVE generation; the invariant companion enforces
messageSeqs-empty iff user-source on every appended session/title event
(tsconfig gains the session-title invariant path); SessionTitleEventData
field docs state the third source kind and the empty-seqs rule, mirrored
into the bilingual core-data-structures page; the note qualifies the
refresh unpin as conditional on a derivable replacement.

docs/core-data-structures/session-title.md

@@ -45,9 +45,9 @@ type SessionTitleSource =
 interface SessionTitleEventData {
   /** Normalized non-empty title text. */
   readonly title: string
-  /** Exact human `user/message` seqs used to derive this title. */
+  /** Exact human `user/message` seqs used to derive this title; empty for an explicit user rename. */
   readonly messageSeqs: number[]
-  /** Built-in fallback or registered-provider provenance. */
+  /** Built-in fallback, registered-provider, or explicit-user provenance. */
   readonly source: SessionTitleSource
 }
 ```