threads(M6): getCurrentId, docs, and CI wiring — threading built

New thread_self=42 syscall backs runtime.Thread.getCurrentId (the calling
thread's kernel task id). thread-test gains an id mode: two workers read
getCurrentId and the main thread confirms all three ids are non-zero and
distinct. Per-thread threadlocal TLS is deferred by design (no consumer; it would
need context-switched fs.base for an unused feature), as are RwLock/WaitGroup.

Marks the threading feature built (M1-M6): threading.md + docs/README.md status
updated, all thread-* cases wired into qemu_test.py.

Gate thread-id PASS; full suite green: 21/21 (thread-spawn/join/futex/mutex/id,
aspace-refcount, + 15 guardrail cases), zig build clean, zig build test green.

system/services/thread-test/thread-test.zig

@@ -257,6 +257,43 @@ fn runMutexMode() void {
     write("thread-mutex: ok\n"); // the M5 verdict marker
 }
 
+// --- M6: id mode (getCurrentId identity) ------------------------------------
+
+var worker_ids: [2]std.atomic.Value(u32) = .{ std.atomic.Value(u32).init(0), std.atomic.Value(u32).init(0) };
+
+fn idWorker(slot: usize) void {
+    worker_ids[slot].store(runtime.Thread.getCurrentId(), .release);
+}
+
+fn runIdMode() void {
+    write("thread-id: starting\n");
+    const main_id = runtime.Thread.getCurrentId();
+
+    const t0 = runtime.Thread.spawn(.{}, idWorker, .{@as(usize, 0)}) catch {
+        write("thread-id: FAIL spawn\n");
+        return;
+    };
+    const t1 = runtime.Thread.spawn(.{}, idWorker, .{@as(usize, 1)}) catch {
+        write("thread-id: FAIL spawn\n");
+        return;
+    };
+    t0.join();
+    t1.join();
+
+    const id0 = worker_ids[0].load(.acquire);
+    const id1 = worker_ids[1].load(.acquire);
+    // Each thread has its own kernel task id: all three distinct and non-zero.
+    if (main_id == 0 or id0 == 0 or id1 == 0) {
+        write("thread-id: FAIL a thread reported id 0\n");
+        return;
+    }
+    if (id0 == id1 or id0 == main_id or id1 == main_id) {
+        write("thread-id: FAIL thread ids collided\n");
+        return;
+    }
+    write("thread-id: ok\n"); // the M6 verdict marker
+}
+
 pub fn main(init: runtime.process.Init) void {
     const mode = init.arguments.get(1) orelse "spawn";
     if (std.mem.eql(u8, mode, "join")) {
@@ -265,6 +302,8 @@ pub fn main(init: runtime.process.Init) void {
         runFutexMode();
     } else if (std.mem.eql(u8, mode, "mutex")) {
         runMutexMode();
+    } else if (std.mem.eql(u8, mode, "id")) {
+        runIdMode();
     } else {
         runSpawnMode();
     }