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.

tsconfig.base.json

@@ -86,6 +86,7 @@
         "./packages/hooks/*/src/invariant.ts",
         "./packages/session-persistence/*/src/invariant.ts",
         "./packages/session-projection/*/src/invariant.ts",
+        "./packages/session-title/*/src/invariant.ts",
         "./packages/session-query/*/src/invariant.ts",
         "./packages/telemetry/*/src/invariant.ts",
         "./packages/acp/*/src/invariant.ts",