perf: reduce scheduler mutex contention + stack pool
Three changes, each independently measured, landed together. Fuse pre-resume mutex acquisitions ----------------------------------- The schedule loop previously took the shared lock three separate times per actor resume: once to pop the run queue, once to read the stack pointer, and once to pop the first-resume closure. These are now a single acquisition that returns everything needed to resume the actor. As a side effect, pending_closures was changed from a Vec<(Pid, Closure)> with O(n) linear scan to a Vec<Option<Closure>> indexed by slot index, making first-closure lookup O(1). Move stack allocation outside the shared lock ---------------------------------------------- Stack::new() (mmap + mprotect) was previously called inside with_shared, stalling every other scheduler thread for the duration of two syscalls on every spawn. It now runs before the lock is acquired. Stack pool ---------- Rather than munmap-ing a stack when an actor finishes and mmap-ing a fresh one on the next spawn, stacks are now recycled through a per-Runtime pool (Mutex<Vec<Stack>>). finalize_actor extracts the stack from the Actor before clearing the slot and pushes it to the pool outside the shared lock. spawn_under pops from the pool before falling back to Stack::new(). The pool is unbounded for now (shrink policy TBD) but capped at stack_pool_cap stacks on return, defaulting to thread_count * 4. The cap is configurable via Config::stack_pool_cap(n). Results (24-thread, against stored baseline) -------------------------------------------- chained_spawn smarm 1-thread: 9763 → 261 µs (-97%) chained_spawn smarm 24-thread: 23562 → 838 µs (-96%) ping_pong_oneshot smarm 1-thread: 18409 → 742 µs (-96%) ping_pong_oneshot smarm 24-thread: 44596 → 1425 µs (-97%) catch_unwind_panics smarm 24-thread: 267812 → 124094 µs (-54%) fan_out_compute smarm 24-thread: 2839 → 2226 µs (-22%) Tokio regressions in the checker output are baseline measurement drift; no tokio code was changed.
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