threads(M3): join, detach, and cross-core parallelism

thread_spawn takes a 4th arg, an exit-endpoint handle: spawnThreadSupervised
resolves and refcounts it under the spawn lock (like spawnProcessSupervised), so
a thread's death posts a child-exit notification carrying its tid. runtime
Thread.join blocks in replyWait on that (private) endpoint for its tid, then
munmaps the stack; detach relinquishes the join (stack reclaimed at process
exit, for now). New current_core=39 syscall + Thread.currentCore() lets a worker
observe which core it ran on.

The closure now lives at the top of the thread's own (private) stack instead of
the heap, so spawn/join never touch the not-yet-thread-safe runtime heap.

thread-test gains a join mode: 4 workers x 100k atomic increments, joined, with
counter == N*K and >1 core stamped (real parallelism), plus a detached worker.
Gate thread-join PASS (4x, non-flaky); 17 guardrail/M1/M2 cases green; build +
host tests clean.

system/abi.zig

@@ -63,8 +63,9 @@ pub const SystemCall = enum(u64) {
     shm_create = 34, // shm_create(len) -> vaddr (rax), handle (rdx): a shareable, zeroed, cacheable RAM region mapped into this AS; the handle is a capability passed to another process as an ipc_call send_cap (docs/display-v2.md)
     shm_map = 35, // shm_map(cap) -> vaddr: map the shared region named by a received capability into this AS (the same physical pages the creator sees)
     shm_physical = 36, // shm_physical(cap) -> paddr: the guest-physical base of a shared region held by capability, so a driver can program it into a device (e.g. virtio-gpu attach_backing); the pages are contiguous (docs/display-v2.md)
-    thread_spawn = 37, // thread_spawn(entry, stack_top, arg) -> tid: start a task that shares the caller's address space at `entry` on `stack_top`, with `arg` in rdi (docs/threading.md)
+    thread_spawn = 37, // thread_spawn(entry, stack_top, arg, exit_endpoint) -> tid: start a task sharing the caller's address space at `entry` on `stack_top`, `arg` in rdi; exit_endpoint (a handle, or no_cap) is notified when it ends — how join waits (docs/threading.md)
     thread_exit = 38, // thread_exit(): end the calling thread, dropping one reference to its address space (destroyed on the last)
+    current_core = 39, // current_core() -> index: the dense 0-based index of the core the caller is running on (for parallelism/affinity introspection)
     _,
 };