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.
The default arm of copySessionTitleSource is a closed-union exhaustiveness
guard (compile-time protection for the next kind); it is unreachable at
runtime, so the per-file coverage gate ignores it like every other
assertNever arm.
sessionTitle.rename appends a user-source session/title event; onUserMessage
skips scheduling while a user title stands, and an explicit refresh is the
deliberate unpin (provider regeneration, or a re-derived fallback when no
provider is registered).
