runtime.Thread.RwLock (reader-preferring, lock/tryLock/unlock +
lockShared/tryLockShared/unlockShared) and WaitGroup (start/finish/wait), both on
the existing Mutex/Condition.
A compile-time Futex seam gated on builtin.os.tag: the futex syscalls on danos, a
spin+yield mock off-target (Zig 0.16 has no std.Thread.Futex; wake is a no-op
since the state machines re-check). thread.zig is wired into zig build test, so
Mutex/RwLock/WaitGroup run as host unit tests with real std.Thread threads (test
blocks compile only under test, so std.Thread there is fine on freestanding).
thread-rwlock QEMU case: 2 writers set both halves of a value under the exclusive
lock while 3 readers check they match under the shared lock; zero half-write
observations across ~150k reads.
Marks Phase 2 (M7-M11) built. threading.md/threading-plan.md status updated.
Gate: host zig build test covers the sync primitives; thread-rwlock PASS (3x);
full Done gate 26/26 (whole thread-* suite + guardrail); build clean.
daniel
a4e44e8f31