The incremental-append path groups the appended region before the file's
own cached keys join the dedup set, so a streamed assistant id restated
across the append boundary counted twice, and the boundary merge could
splice it into the wrong turn (reproduced on real image-heavy sessions:
one id streamed across several records over seconds, 111 calls vs the
110 a full re-parse yields). Suppressing the restatement instead would
freeze the stale first emission, so neither shortcut matches a full
re-parse. On any id overlap between the appended entries and the cached
turns the shortcut is abandoned and the file re-parses from byte 0.
Rare (about 0.3 percent of real files) and bounded to one file per
refresh. Regression test uses the cold-reparse oracle; mutation-verified
against the unguarded code.
Co-authored-by: reviewer <review@local>
