The fan-out path used to report usage as if a single Codex call had
run: `prompt_tokens = max(1, len(prompt)//4)` and
`completion_tokens = len(synthesis)//4`. In reality it had spawned
N parallel worker turns (each carrying the same prompt) plus a
synthesis turn that re-sent the prompt and every tab's output. For
`parallel_calls=20` that meant the cost / context widget under-
reported the request by roughly 20x.
Now sums:
- `prompt_tokens ≈ (N * prompt + synthesis_prompt) / 4` where
`synthesis_prompt = sum(tab_outputs) + prompt`.
- `completion_tokens ≈ (sum(tab_output_chars) + synthesis_chars) / 4`.
Tests: new regression
`test_parallel_usage_accounts_for_all_calls` runs a 4-way fan-out
against a fake SDK with deterministic chunk lengths and asserts
the reported usage scales with N, not the single-call shape.
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