ERR/E3.1: thread-local error return-trace ring buffer (runtime)

Add the trace buffer that raise/try push to and catch/or/destructure clear,
following the JNI-TLS precedent exactly (a thread_local IR global doesn't work
under the ORC JIT, which doesn't init TLS for AddObjectFile'd objects).

- library/vendors/sx_trace_runtime/sx_trace.c: a `_Thread_local` fixed-cap ring
  (32 frames) of opaque u64s + C API (push / clear / len / truncated /
  frame_at). Overflow keeps the newest CAP frames and latches `truncated`
  (Zig-style); frame_at returns oldest-to-newest. The frame is opaque — the
  E3.3 formatter dispatches on context (PC at runtime, packed (func_id, offset)
  at comptime).
- build.zig: link the .c into the compiler so the JIT resolves sx_trace_* via
  dlsym (and so the unit test links against it).
- src/runtime_trace.test.zig: exercises push / overflow-survives-newest / clear
  / len / truncated / ordering against the linked C — grounds the buffer logic
  without shipping throwaway sx builtins.
- lower.zig getTraceFids(): lazily declares the sx_trace_push/clear externs +
  sets needs_trace_runtime. Declared now; the raise/try push sites and the
  absorbing clear sites get wired at E3.2.
- core.zig: auto-injects the .c as a #source for AOT when needs_trace_runtime,
  mirroring the JNI env runtime.

Gates: zig build, zig build test (incl. the new buffer tests), bash
tests/run_examples.sh (277 passed; no codegen change this step — lone failure
is the user's uncommitted 213-canonical-map pack WIP).

build.zig

@@ -36,6 +36,14 @@ pub fn build(b: *std.Build) void {
         .file = b.path("library/vendors/sx_jni_runtime/sx_jni_env_tl.c"),
         .flags = &.{},
     });
+    // ERR E3.1 runtime — `_Thread_local` error return-trace ring buffer.
+    // Same linkage rationale as the JNIEnv* slot above: linked into the
+    // compiler so the JIT resolves `sx_trace_*` via dlsym; AOT outputs pick it
+    // up via a lower-side auto-injected c_import (gated on needs_trace_runtime).
+    mod.addCSourceFile(.{
+        .file = b.path("library/vendors/sx_trace_runtime/sx_trace.c"),
+        .flags = &.{},
+    });
     mod.addCSourceFile(.{
         .file = b.path("clang_shim.cpp"),
         .flags = &.{