ci: replace retired macos-13 runner with macos-15/macos-15-intel

GitHub retired the macos-13 image. Both darwin matrix jobs failed
instantly on v0.1.5 ("configuration 'macos-13-us-default' is not
supported") and hang unschedulable on v0.1.6.

x86_64-apple-darwin -> macos-15-intel
aarch64-apple-darwin -> macos-15 (native ARM64)

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018e5FTMgwzBD46RZaN8z8An

Author: srv1n

.github/workflows/release.yml

@@ -186,12 +186,12 @@ jobs:
 
           # macOS builds
           - target: x86_64-apple-darwin
-            os: macos-13
+            os: macos-15-intel
             name: darwin-intel
             display_name: "macOS Intel"
             cross: false
           - target: aarch64-apple-darwin
-            os: macos-13
+            os: macos-15
             name: darwin-apple-silicon
             display_name: "macOS Apple Silicon"
             cross: false