Two internal-consistency fixes from the independent design review:
the 429 saturation argument wrongly claimed exponential backoff growth
(429 skips the retry budget, so max(fails, 1) pins the delay to the base
tier), and "productive" was defined as waiting on the response while the
StallClock actually wraps all of _attempt. The boundary is now stated as
_attempt itself, including per-attempt accounting and telemetry, with the
rationale that telemetry jitter must not participate in the stall verdict.
iomgaa
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