threads(M10): per-thread fs.base — the TLS thread-pointer mechanism

Each thread gets its own x86_64 thread pointer (FS base) for user-space TLS.
Task.fs_base is restored on every context switch only when it changes (same
conditional-load discipline as CR3; architecture.setFsBase -> wrmsr
IA32_FS_BASE). New set_thread_pointer=44 syscall sets the caller's fs_base and
loads it now. The kernel never touches FS, so no swapgs complication.

The runtime lays a small per-thread TLS block at the top of each thread's stack
(self-pointer at %fs:0 + scratch) and the thread trampoline calls
set_thread_pointer before any user code — so every spawned thread has a private,
switch-stable thread pointer, reclaimed with the stack.

thread-test tls mode: two threads write unique markers to their own %fs:8 and,
after both wrote, read back — a shared fs.base would clobber one (cross-talk).

Deferred: the Zig threadlocal *compiler* layer (ELF variant-II PT_TLS + linker
sections + template copy) — high-uncertainty, no consumer today; this lands the
load-bearing per-thread fs.base it builds on. See docs/threading-plan.md M10.

Gate thread-tls PASS (3x); full guardrail 25/25; build + host tests clean.

system/kernel/architecture/x86_64/cpu.zig

@@ -304,6 +304,15 @@ pub fn cpuLocal() usize {
     return pcpu.scheduler();
 }
 
+const ia32_fs_base = 0xC000_0100;
+
+/// Set the FS-segment base — the x86_64 thread pointer for user-space TLS. The kernel
+/// never touches FS, so this only affects the user task that runs next; the scheduler
+/// restores it per task across context switches (docs/threading-plan.md M10).
+pub fn setFsBase(base: u64) void {
+    io.wrmsr(ia32_fs_base, base);
+}
+
 // --- SMP: application-processor bring-up ----------------------------------
 
 /// Record the low (<1 MiB) frame reserved for the AP trampoline. Run once at boot.