streaming: add stream TTL (absolute and sliding)

[raphael]

What

This adds built-in TTL support for Redis streams in Pulse, configurable via stream options:

• options.WithStreamTTL(ttl): absolute TTL (set once when the Redis key is created; never extended)
• options.WithStreamSlidingTTL(ttl): sliding TTL (refreshed on each publish)

TTL is applied in both stream creation paths:

• Publish path (Stream.Add / XADD) — after the write succeeds
• Sink path (newSink / XGROUP CREATE ... MKSTREAM) — after the group/stream exists

Docs: added a short “Stream retention (TTL)” section to streaming/README.md.

Why

Many production uses want automatic retention for per-run/per-session streams to avoid leaked keys accumulating indefinitely. Pulse already caps stream length via MaxLen, but length trimming alone doesn’t guarantee eventual cleanup of inactive streams.

How

• Stream options now carry TTL + TTLSliding.
• Stream.Add and newSink call a shared internal helper applyTTL:
  • Absolute TTL uses ExpireNX (set only if no TTL exists).
  • Sliding TTL uses Expire (refreshes TTL).
• NewStream fails fast on negative TTL (ttl < 0).

Behavioral notes

• TTL starts when the Redis stream key is first created, which can be:
  • first publish (XADD), or
  • sink creation (MKSTREAM) even before the first event.
• Default remains no TTL unless an option is set.

Test evidence

• go test ./streaming/options -run TestStreamOptions
• go test ./streaming -run TestOptions

TTL behavior tests live in streaming/streams_test.go and require a Redis instance (e.g. localhost:6379).