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.

library/runtime/thread.zig

@@ -100,6 +100,11 @@ pub const Thread = struct {
         _ = self;
     }
 
+    /// The calling thread's id (its kernel task id). Mirrors `std.Thread.getCurrentId`.
+    pub fn getCurrentId() Id {
+        return @intCast(sc.systemCall0(.thread_self));
+    }
+
     /// The dense 0-based index of the core the calling thread is running on. A danos
     /// extension beyond `std.Thread`, used to observe genuine cross-core parallelism.
     pub fn currentCore() Id {