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/kernel/scheduler.zig

@@ -424,17 +424,6 @@ pub fn spawnUserLocked(aspace: u64, entry: u64, user_sp: u64, user_arg: u64, pri
     return t.id;
 }
 
-/// Spawn a **thread**: a user task that shares an *existing* address space `aspace`
-/// (docs/threading.md), starting at `entry` on `user_sp` with `arg` delivered in its
-/// rdi. Takes a reference to `aspace` (destroyed only when the last thread on it
-/// exits). Acquires the kernel lock itself. `supervisor` is the spawning process.
-/// Returns the new thread's id, or null if the task table is full / out of memory.
-pub fn spawnThread(aspace: u64, entry: u64, user_sp: u64, arg: u64, priority: Priority, supervisor: u32) ?u32 {
-    const flags = sync.enter();
-    defer sync.leave(flags);
-    return spawnUserLocked(aspace, entry, user_sp, arg, priority, "thread", supervisor, null);
-}
-
 /// The first thing a fresh user task runs (in ring 0, via task_trampoline). It
 /// drops to ring 3 at the task's recorded entry/stack. Reading them from the
 /// Task avoids smuggling values through callee-saved registers across the