docs+code: drop misleading fs.base notation for the thread pointer

The "fs.base" spelling read like a field/submodule access, but no such
identifier exists — it meant the x86_64 FS segment base (the IA32_FS_BASE
MSR). Two problems fixed:

- Arch-neutrality: in generic docs, the runtime, and the plan, the mechanism
  is now named by its arch-neutral concept — the "thread pointer" — matching
  the already-renamed `thread_pointer` Task field, `set_thread_pointer`
  syscall, and `architecture.setThreadPointer` fn. x86-specific spots keep
  the precise names: `IA32_FS_BASE` (the MSR), `%fs:8`/`%fs:0`, variant-II.

- Stale identifiers: the M10 section in threading-plan.md still referenced
  `fs_base` on Task and `architecture.setFsBase` — both renamed away in the
  arch-neutral pass. Corrected to `thread_pointer` / `setThreadPointer`.

The x86-only `thread-tls` test (which really does write `%fs:8`) now says
"FS base" (no dot) consistently, matching the established form already in
tests.zig. The matched serial markers ("thread-tls: ok" / "thread-tls:
FAIL") are unchanged; only a non-load-bearing FAIL parenthetical was
reworded.

Verified: zig build clean, thread-tls passes.

library/runtime/thread.zig

@@ -60,7 +60,7 @@ pub const Thread = struct {
             /// thread's TLS pointer, runs the user function, then ends the thread.
             fn entry(self_addr: usize) callconv(.c) noreturn {
                 const self: *@This() = @ptrFromInt(self_addr);
-                setThreadPointer(self.tls_base); // per-thread FS base before any user code
+                setThreadPointer(self.tls_base); // per-thread thread pointer before any user code
                 @call(.auto, function, self.args);
                 exitThread();
             }
@@ -70,7 +70,7 @@ pub const Thread = struct {
         if (system.mmapFailed(base)) return error.SystemResources;
 
         // Top of the thread's own stack, downward: the closure, then a small per-thread TLS
-        // block (fs.base points here; slot 0 is the variant-II self-pointer, the rest is
+        // block (the thread pointer points here; slot 0 is the variant-II self-pointer, the rest is
         // scratch for user TLS), then the stack proper (rsp starts below the TLS block, so
         // the growing stack never overwrites either).
         var closure_addr = (base + config.stack_size) - @sizeOf(Closure);