[gschaetz]

Implements the spec merged in #114 (add-retry-after), step 1 of Replacing ModelRelay. No release is cut here.

Behavior

• A 429 or 503 whose error body states a retry time ("Please try again in 7.66s", retryDelay, retry_after, ...) cools the endpoint at once for exactly that long, clamped to 1 second .. [health] max_retry_after_secs (new, default 300, validated 1 to 86400). It does not wait for failure_threshold, does not double, is not held to max_cooldown_secs, and does not count as a trip, so a later unannounced failure streak is not escalated by it.
• Anything unreadable, or any other status, behaves exactly as before. Only the first 4 KiB of the body is read, a bounded duration is the only output, and nothing from the body is logged or stored.
• One seam, retry_after::from_body, where the Retry-After header joins once Switchyard exposes it (fix(server): preserve safe headers on terminal upstream errors NVIDIA-NeMo/Switchyard#904, which I added a use case to).
• Visible: cooldown_source (retry_after or failures) in GET /v1/health and route/explain; retry_after_ms on decision attempts (inside the existing attempts column, no schema change); counter humpyard_retry_after_honored_total{provider,source}.
• Health still never denies service: a retry time only cools endpoints, and the fail-open pass is unchanged (covered by a test).

Tests

Parser unit tests (phrasings, JSON fields, units and combinations, bare numbers, junk, zero and negative, a day, a body past 4 KiB, a multibyte character at the limit), breaker unit tests (exact duration, no doubling or cap, late failures, failed probe, no escalation, disabled breaker), and 9 end-to-end tests with a controllable clock: cooled at once for the stated time and probed exactly then, the cap, the one-second floor, 503, hint-less failures unchanged, a 500 mentioning a time ignored, metrics and decision fields, fail-open, config validation. The architecture-rules test gains the new module. Local gates: fmt, clippy -D warnings, 452 tests, cargo deny.

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