feat(core): live actor introspection streaming (v1.1)

Per spec section 3.2, add streaming metrics API for sub-millisecond
freshness debugging of stalled NSAI reasoning chain actors.

New types (additive — existing pull API unchanged):
- LiveMetrics — observation record (HLC ts, queue depth, EWMA rates,
  p50/p99 latency, state size, GPU util, tenant_id)
- IntrospectionStream — subscription dispatcher with per-actor and
  global buckets via tokio mpsc::unbounded_channel; auto-prunes broken
  receivers; monotonic subscription IDs
- SubscriberHandle — interval-gated sender (slow subscribers safe)
- MetricAggregator — Send+Sync with configurable EWMA alpha (default
  0.2), seeds EWMA with first observation, consumes deltas on snapshot
- LatencyHistogram — 1024-slot ring buffer with sort-on-read p50/p99
- SubscribeMetricsRequest (24B, #[repr(C)]) — H2K wire format
- LiveMetricsEvent (72B, #[repr(C)] w/ explicit padding) — K2H wire format
- Compile-time size assertions for ABI stability

42 new tests + 5 pre-existing = 47 introspection tests passing.
782 ringkernel-core lib tests pass total.

Backward compatible: ActorIntrospection pull API untouched, tick()
support for coordinator calls, all existing tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mivertowski