feat: honor Retry-After, jitter backoff, bound response-header wait

[acoshift]

The cheap reliability bundle from the review (#2, #4, #3). All in the flush/retryFlush/transport path next to the taxonomy from #21.

429 / Retry-After (#2)

flush() now reads Retry-After on a 429 (or 503), parses both delta-seconds and HTTP-date forms, clamps to RetryAfterMax (60s), and paces the next retry to it instead of the fixed 100ms–1s backoff. The value is carried to retryFlush via a worker-local, and the retry sleep is now a closeSignal-interruptible timer, so a long Retry-After (or a Close) can't delay shutdown. Retry-After is intentionally ignored on the closing path so a server-requested delay can't stretch shutdown.

→ Stops the client re-POSTing a saturated Quickwit ingest queue once/sec and lets it cooperate with server-side pacing.

Backoff jitter (#3)

Both retry loops use equal jitter (sleep in [d/2, d], via math/rand/v2 — no new dep, concurrency-safe). De-synchronizes retries across workers so they don't arrive in lockstep waves that can re-tip a recovering server.

ResponseHeaderTimeout (#4)

The default transport bounds the wait for response headers to getIngestTimeout()/3 (~5s default). A server that completes the request but stalls before responding (typical L4 LB fronting an overloaded indexer) frees the worker in ~5s instead of the full 15s deadline. Firing returns a transport error, which is already retryable. User-supplied clients via SetHTTPClient are untouched.

Invariants

429/503 stay retryable; the failure taxonomy, fire-and-forget batching, ordering, and the at-least-once/exactly-once-settle guarantees are unchanged. retryAfter is a worker-local (one per loop() goroutine), so no new shared state.

Tests

• Unit: parseRetryAfter (seconds / HTTP-date / past-date / empty / garbage / zero / negative), jitterBackoff bounds over 1000 draws, ResponseHeaderTimeout == ingestTimeout/3.
• End-to-end: a 429 with Retry-After: 1 paces the retry to ~1s (vs the 100ms backoff) and still delivers; held over -count=4 -race.

Full suite green, go vet clean, race-clean across -count=2.

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