chore: expand CI matrix to full OTP × Elixir lattice (#17)

Test all four combinations: OTP 27/28 × Elixir 1.18/1.19, not just
the diagonal. Catches compatibility issues when versions are crossed.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cigrainger

.github/workflows/ci.yml

@@ -17,11 +17,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        include:
-          - otp: "27"
-            elixir: "1.18"
-          - otp: "28"
-            elixir: "1.19"
+        otp: ["27", "28"]
+        elixir: ["1.18", "1.19"]
 
     steps:
       - uses: actions/checkout@v4