`SESSION_FORMAT_VERSION` is the only value that names the Session format
generation. Two other version fields had moved to 2 alongside it:
- The `dsh_session_log` request extension's outer schema `version` returns
to 1 and its Session header projection keeps `seedLength`, derived from
the logical inherited cut, so the external wire is unchanged in this PR;
`sessionFormatVersion` alone identifies the embedded generation.
- The `sessionStats` projection unit's `stateVersion` returns to 1: the
projection cache binds every checkpoint to the format generation, so a
generation change discards old rows without a unit version bump.
Test doubles and the standalone Client fixture that build a current
header now spell `SESSION_FORMAT_VERSION` instead of the literal.
The `start` frame and the reconnect baseline attempt carried a wall-clock
`startedTime` that no Host or Client consumer read: the Host accumulator
and the Client reconciler copied it into their attempt maps and nothing
looked at it again. Presentation frames now carry only the coordinates
consumers use (attempt, revision, turn, step, chunk index, and v1 seq
provenance); the type-equivalence block, event graph, READMEs, and the
live-stream Agent Note follow the type.
Single RemoteError with a merge-extensible, domain-prefixed code map;
owners throw at the failure point; streams surface marked failures;
clients consume ctx.remote directly with isRemoteFailure as the only
discrimination point and construct no failure instances.
Tianyi Cui