Add multi-version Go test matrix

Test against Go 1.15 through 1.24 to verify backward compatibility
and identify version-specific issues.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: oliveagle

.github/workflows/ci.yml

@@ -9,11 +9,15 @@ on:
 jobs:
   test:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        go-version: ['1.15', '1.16', '1.17', '1.18', '1.19', '1.20', '1.21', '1.22', '1.23', '1.24']
+      fail-fast: false
     steps:
       - uses: actions/checkout@v4
-      - name: Set up Go
+      - name: Set up Go ${{ matrix.go-version }}
         uses: actions/setup-go@v5
         with:
-          go-version: '1.15'
+          go-version: ${{ matrix.go-version }}
       - name: Test
         run: go test ./...