Static delay: SDK-level persistence hook, mandatory range validation

[balloob]

This issue comes out of a cross-SDK conformance audit comparing every Sendspin client/server implementation (aiosendspin, sendspin-cli, sendspin-cpp, sendspin-dotnet, sendspin-go, sendspin-js, sendspin-jvm, sendspin-rs, SendspinKit) against the spec. sendspin-cpp is treated as the reference implementation throughout — it most closely matches the spec's language and is the only SDK that runs on the constrained-embedded target the spec was originally written for.

static_delay_ms is a small but spec-mandatory field, and persistence
is broken almost everywhere: only sendspin-cpp and sendspin-cli
persist the value. Two SDKs (sendspin-rs and SendspinKit) parse
the value but never apply it in playback — the highest-severity bug
the audit found (already filed against those SDKs). sendspin-cpp is
the reference here: SendspinPersistenceProvider for storage,
sync_task.cpp:231 subtracts effective_static_delay_ms × US_PER_MS
from converted timestamps, and set_static_delay_adjustable(bool)
supports per-output values.

1. Restate persistence as a recommended SDK feature

Today the spec text "Clients must persist static_delay_ms locally"
reads as a requirement on the device. In practice the SDK is where
that persistence has to live. Add:

Client SDKs SHOULD provide a persistence hook for static_delay_ms
(file, key-value, or callback) so applications need not
re-implement storage.

Point to sendspin-cpp's SendspinPersistenceProvider as the
reference shape.

2. Make range validation MUST

Clients MUST clamp incoming static_delay_ms values to 0–5000 and
MUST treat any out-of-range value as a protocol error to log.

sendspin-cpp enforces 0–5000 strictly; sendspin-cli silently
coerces negatives to positives; sendspin-js clamps; the rest don't
check at all. A misbehaving server or local config could supply
out-of-range values today.

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Source audit doc: docs/static-delay.md
Full audit branch: claude/stream-sync-correction-sdks-AWoNC
Per-SDK digest: sdk-issues digest