The streaming hot loop computed len(repr(chunk)) on EVERY chunk to feed
the retry-diagnostic byte counter — a full recursive pydantic repr at
5.5-8.8 us per chunk (measured), ~20-30 ms of pure CPU per 3,000-chunk
response, paid on every streaming response on every platform.
New _estimate_chunk_bytes() sizes the chunk from its delta payload
strings (content / reasoning / tool-call arguments) plus a 40-byte
framing floor: 2.1-2.4 us per chunk (~3x cheaper), independent of
pydantic field count, never raises on unknown shapes (Anthropic events,
stub providers fall back to the floor). Both call sites switched
(chat-completions loop + anthropic event loop).
The counter feeds only the stream-retry diagnostic log line
(agent/stream_diag.py) — an estimate proportional to traffic preserves
its purpose (distinguishing 'died at 0 bytes' from 'died mid-stream').
6 new contract tests; 64 targeted stream tests green.
