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).
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