Fixes two related native-streaming bubble defects surfaced in production:
1. Duplicate bubble on long turns — when keep-alive already refreshed the
6-min reply window, the Layer-2 clock fallback still declined the finalize
frame and forced a proactive send(), duplicating the message. Skip the
clock fallback while keep-alive is active; intermediate-frame failures are
now fully fire-and-forget (only a failed FINAL frame falls back to send()).
2. Split / mini bubbles ('Cla' + 'ude ...') — two compounding root causes:
a) In native streaming a mid-turn commentary (e.g. a Hindsight recall
notice) called _reset_segment_state(), clearing the cumulative
_accumulated so the next delta + finalize frame carried only the few
chars accumulated after the reset. Native streaming now skips that
reset (commentary still posts as its own message via send()).
b) The adapter-side _BlockChunker.update() 'only grow' guard silently
dropped any cumulative snapshot shorter than its high-water mark, so
after a baseline reset the leading characters were stranded before
_emitted_len. Removed the _BlockChunker sentence-alignment + idle-flush
layer entirely; intermediate frames are pure identity-dedup, matching
the fire-and-forget model.
Also removes ~232 lines of now-dead code (_BlockChunker class, idle-flush
machinery, block-stream constants) and aligns the test suite with the
fire-and-forget frame model, including a regression test that locks the
native-commentary-no-reset behavior.
Tests: 177 passed, 3 skipped (wecom + stream_consumer suites).
wansui
9faa953cd1