ci(zig-ffi): swap retired macos-13 runner for macos-15-intel (#29)

## Summary
- `macos-13` GitHub-hosted runner image was deprecated 2025-09-22 and
**fully unsupported from 2025-12-04** (actions/runner-images#13046,
[#13402](actions/runner-images#13402),
[#13634](actions/runner-images#13634)). The
Intel `x86_64-macos` matrix leg of `zig-ffi.yml` (which uses `runs-on:
${{ matrix.os }}` with `os: macos-13`) now queues forever — there is no
runner — with no error surface.
- Swap `os: macos-13` → `os: macos-15-intel` (current Intel x64 standard
runner per the actions/runner-images README). macOS 15 is the safer of
the two Intel-x64 options: stable GA image, vs `macos-26-intel` which
only graduated from public beta in 2026-02 and had an arm-label
regression in actions/runner-images#14112 as recently as 2026-05-19.
- The `target: x86_64-macos` artefact name stays the same — only the
build host moves.

## Estate context
Same one-line swap landed on hyperpolymath/affinescript today (PR
hyperpolymath/affinescript#292, merged commit `26cdb43`), where the
retired-runner trap was first diagnosed after a release run sat queued
~10h with no error. Propagating the fix here.

## Test plan
- [ ] Reviewer to confirm macos-15-intel is permitted under any local
runner allowlist (governance.yml if present).
- [ ] Next push that triggers zig-ffi.yml picks up an Intel macOS runner
and completes the x86_64-macos build.

Refs hyperpolymath/affinescript#292, actions/runner-images#13046.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/zig-ffi.yml

@@ -75,7 +75,7 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - os: macos-13
+          - os: macos-15-intel
             target: x86_64-macos
           - os: macos-14
             target: aarch64-macos