feat: named RetryCurve API for switching retry regimes at runtime

[tanderson-ld]

Summary

Adds a RetryCurve API so a caller can register multiple retry-timing curves on a single stream and switch between them at runtime via Stream.ActivateCurve. Enables consumers to run a multi-regime retry policy (e.g., a normal regime + an extended regime for auth failures) while keeping the library's single-regime timing path intact for existing callers.

Marked draft for API/design socialization before dependent SDK work builds on it.

API additions

• NewRetryCurve(options ...RetryCurveOption) *RetryCurve — construct an opaque handle.
• RetryCurveBaseDelay(d), RetryCurveMaxDelay(d), RetryCurveJitter(r) — curve options.
• StreamOptionDefaultRetryCurve(curve) — designate as the stream's effective default.
• StreamOptionRegisterRetryCurve(curve) — register as an additional switchable curve.
• Stream.ActivateCurve(curve *RetryCurve) — switch the currently-active curve at runtime.
• DefaultCurve — package-level sentinel meaning "revert to the effective default."
• MaxServerDirectedRetryDelay = time.Hour — clamp ceiling for the SSE retry: field.

Legacy stream options (StreamOptionInitialRetry, StreamOptionUseBackoff, StreamOptionUseJitter, StreamOptionRetryResetInterval) continue to work unchanged; when no explicit RetryCurve is provided they synthesize the effective default.

Semantics

• Lazy overlay resolution. At delay-computation time, each property is resolved by walking active-curve.spec → effective-default.spec → hard-coded fallback. Curve specs are immutable.
• Per-curve formula counter n. Each registered curve tracks its own backoff-formula counter. Progression is retained across activations, so rapid oscillation between regimes preserves each regime's state.
• Healthy-op reset. When elapsed >= resetInterval, zeros every curve's n and reverts active to the effective default. Does NOT clear server-directed base-delay overrides (matches HTML5 SSE spec's "reconnection time is set until updated").
• Server retry: field. Updates every registered curve's base-delay override (stream-wide per HTML5). Never touches any curve's declared maxDelay ceiling.
• Wire clamp. SSE retry: values above MaxServerDirectedRetryDelay are clamped, in milliseconds before the time.Millisecond multiplication, so extreme int64 wire values cannot overflow the Duration.

Internal notes for reviewers

• backoffStrategy.applyBackoff and jitterStrategy.applyJitter interfaces were widened to accept per-call maxDelay / ratio. Math bodies are unchanged from the pre-existing library; only the parameter source moved from receiver fields to method args, so one strategy instance can serve multiple curves.
• Internal SetBaseDelay renamed to ApplyRetryTime; it now iterates all registered curves.

Test plan

• Full test suite passes (go test ./...).
• SSE contract-test harness passes (make contract-tests — "All tests passed").
• Pre-existing backoff/jitter math bodies preserved byte-for-byte in effect (only parameter sourcing changed).
• Library-maintainer review.

Refs SDK-2788.