feat(scheduler,gen_server,gen_statem,introspect): SpawnOpts — per-actor stack shape on every spawn surface (RFC 019 §1)

SpawnOpts { stack_reserve, guard_size } with Option<usize> fields, None
resolving to the Config defaults at spawn time — a deliberate deviation
from the RFC's plain-usize struct so struct-update syntax works without
a runtime handle in scope. Threaded across the five surfaces:
spawn_with, spawn_under_with, spawn_addr_with,
GenServerBuilder::stack_opts (mirrored on NamedGenServerBuilder), and
gen_statem::spawn_with (gen_statem has no builder, so the opts ride a
_with variant — Claude-solo surface call, flagged for review). Existing
spawns forward defaults; no call-site churn.

introspect::stack_shape(pid) pulled forward (agreed) as the first slice
of the RFC 019 introspection surface, giving tests an observable.

Tests (tests/spawn_opts.rs): override/partial-override/rounding on each
surface; obligation 4 from the outside — a dead custom stack is never
handed to the next default spawn (LIFO pool would expose it), and the
reverse (default stacks ARE recycled); 8 MiB reserve behaviorally
permits ~1 MiB recursion. Also: silence unused-Result in the c1
runtime test (join now unwrapped).

tests/runtime.rs

@@ -545,7 +545,9 @@ fn config_stack_reserve_permits_deep_recursion() {
         spawn(move || {
             std::hint::black_box(burn_stack(64));
             done2.store(true, Ordering::SeqCst);
-        }).join();
+        })
+        .join()
+        .unwrap();
     });
     assert!(done.load(Ordering::SeqCst));
 }