feat: Pin Elixir 1.19.5 and Erlang 28.3 in Nix flake

Add Nix overlay to pin specific versions matching .tool-versions:
- Erlang: OTP 28.3 (via erlang_28)
- Elixir: 1.19.5-otp-28 (via beam.packages.erlang_28.elixir_1_19)

This ensures the Nix build environment matches the asdf-based
development setup exactly, providing consistency across all
development and build environments.

flake.nix

@@ -27,6 +27,21 @@
           config = {
             allowUnfree = false;
           };
+          overlays = [
+            # Pin specific Elixir and Erlang versions to match .tool-versions
+            # Elixir 1.19.5-otp-28, Erlang 28.3
+            (final: prev: {
+              # Use Erlang 28 (OTP 28.3)
+              erlang = prev.erlang_28;
+
+              # Use Elixir 1.19 with Erlang 28 (1.19.5-otp-28)
+              elixir = prev.beam.packages.erlang_28.elixir_1_19;
+
+              # Ensure beam packages use our pinned versions
+              # This affects mixRelease and all BEAM-related builds
+              beamPackages = prev.beam.packages.erlang_28;
+            })
+          ];
         };
 
         packages = {