Adds direct coverage for `Session.requestContext()`: undefined before any
record, folding a seeded log on first read (the watermark starts at 0 with the
seed already in the log, so a first read must consume all of it), incremental
advance across appends, a batch appended between two reads, and the frozen
return that keeps a reader from desyncing later dedup comparisons.
The turn-enclosure assertion goes in the invariant spec rather than beside the
fold tests, because enclosure is enforced by the opt-in invariant companion
plugin and not by `append` itself. Asserting it on a bare Session would have
been a test that passes for the wrong reason.
