fix(ci): move macOS jobs to macos-15 runners

GitHub retires the macos-14 images on 2026-11-02, and its brownouts
already cancel the darwin release builds and the install-script job
(seen on #74). Checked on macOS 15.7 with Apple clang and Rust 1.93.1:
install-sh-test.sh passes with the system's bash and BSD tools, and
both darwin targets build with --locked and run --help (x64 under
Rosetta).

Claude-Session: https://claude.ai/code/session_01RqPPVaoFxLToGAXXBEoCtH

.github/workflows/ci.yml

@@ -43,7 +43,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [macos-14]
+        os: [macos-15]
 
     steps:
       - name: Checkout

.github/workflows/release.yml

@@ -41,10 +41,10 @@ jobs:
           - os: ubuntu-24.04-arm
             target: aarch64-unknown-linux-musl
             asset: linux-arm64
-          - os: macos-14
+          - os: macos-15
             target: aarch64-apple-darwin
             asset: darwin-arm64
-          - os: macos-14
+          - os: macos-15
             target: x86_64-apple-darwin
             asset: darwin-x64