feat(nix): add asciinema, vhs, and cargo-dist to dev shell

Add demo recording tools (asciinema, vhs) and release tooling
(cargo-dist) to nativeBuildInputs. Include a cargo-dist wrapper so
`cargo dist` works as a subcommand. Add scripts/record-demo.sh for
consistent asciinema cast recording at 100x30.

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

flake.nix

@@ -44,6 +44,13 @@
           crate2nix.packages.${system}.default
           pkgs.cargo-expand
           pkgs.cargo-llvm-cov
+
+          # Demo recording and release tooling
+          pkgs.asciinema
+          pkgs.vhs
+          pkgs.cargo-dist
+          # nixpkgs cargo-dist installs as "dist"; alias so `cargo dist` works
+          (pkgs.writeShellScriptBin "cargo-dist" ''exec ${pkgs.cargo-dist}/bin/dist "$@"'')
         ];
         RUST_BACKTRACE = "1";
       };