Limit test-platforms workflow to source file changes

Add path filters to only trigger the workflow when source files are modified:
- lib/** (library code)
- test/** (test files)
- package.json and package-lock.json (dependencies)
- eslint.config.js (linting config)
- .github/workflows/test-platforms.yml (the workflow itself)

This prevents unnecessary workflow runs when documentation files like
README.md or LICENSE are modified.

Author: claude

.github/workflows/test-platforms.yml

@@ -6,9 +6,23 @@ on:
     branches:
       - main
       - 'claude/**'
+    paths:
+      - 'lib/**'
+      - 'test/**'
+      - 'package.json'
+      - 'package-lock.json'
+      - 'eslint.config.js'
+      - '.github/workflows/test-platforms.yml'
   pull_request:
     branches:
       - main
+    paths:
+      - 'lib/**'
+      - 'test/**'
+      - 'package.json'
+      - 'package-lock.json'
+      - 'eslint.config.js'
+      - '.github/workflows/test-platforms.yml'
 
 jobs:
   test-windows: