feat(introspect,runtime): per-actor stack surface on ActorInfo (RFC 019 §8)

- introspect::StackInfo { reserve, guard, depth_high_water,
  parks_since_shrink, shrinks } as ActorInfo.stack; re-exported at crate
  root beside ActorInfo.
- All reads lock-free: geometry from the c6 diag slot atomics, depth =
  top - hwm (the §2 sampled high-water; doc spells out sampled-not-exact
  and that 0 means never-descheduled-at-depth), counters straight off the
  §3 atomics. Coherence for the incarnation rides read_slot's existing
  generation check, same as overruns/messages_received.
- Slot::stack_introspect(): one pub(crate) tuple accessor beside the other
  counter accessors.
- Exact RSS deliberately absent per RFC (mincore = debug tooling only,
  never a runtime path); stack_shape(pid) untouched (cold-lock exact
  variant from c2).
- tests/introspect.rs: defaults surface (64 KiB reserve / 1 MiB guard /
  sampled ~32 KiB depth / gate park counted / zero shrinks) + live shrink
  counters (spike visible pre-shrink; shrinks>=1, cooldown counter reset,
  hwm reset after crossing COOLDOWN) read mid-run -- post-join the slot
  reclaim correctly hides the incarnation, which the first draft of the
  test learned the hard way.

FLAGGED (Claude-solo calls):
- Nested StackInfo struct over five flat ActorInfo fields (grain break;
  the five fields are one concern and ActorInfo is already 12 fields).
- Field names reserve/guard/shrinks (RFC says stack_reserve/stack_guard/
  shrink count; the stack_ prefix is redundant inside StackInfo).

src/lib.rs

@@ -65,7 +65,7 @@ pub use gen_statem::{
     CallError as GenStatemCallError, Cx, Machine, Reply, Resolution, SendError as GenStatemSendError,
     GenStatemRef,
 };
-pub use introspect::{
+pub use introspect::{StackInfo, 
     actor_info, snapshot, tree, tree_from, ActorInfo, ActorState, RuntimeSnapshot, RuntimeTree,
     TreeNode, SNAPSHOT_FORMAT_VERSION,
 };