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/kernel/process.zig

@@ -229,6 +229,7 @@ fn system_call(state: *architecture.CpuState) void {
         .shm_physical => systemShmPhysical(state),
         .thread_spawn => systemThreadSpawn(state),
         .current_core => systemCurrentCore(state),
+        .thread_self => systemThreadSelf(state),
         .futex_wait => systemFutexWait(state),
         .futex_wake => systemFutexWake(state),
         .thread_exit => {
@@ -694,6 +695,11 @@ fn systemCurrentCore(state: *architecture.CpuState) void {
     architecture.setSystemCallResult(state, scheduler.currentCpuIndex());
 }
 
+/// thread_self() -> tid: the calling thread's kernel task id.
+fn systemThreadSelf(state: *architecture.CpuState) void {
+    architecture.setSystemCallResult(state, scheduler.currentId());
+}
+
 /// futex_wait(addr, expected, timeout_ns) -> status (docs/threading.md): if the 4-byte
 /// user word at `addr` still equals `expected`, block until a futex_wake on `addr` or
 /// (if timeout_ns > 0) the deadline. The compare and the block are one critical section,