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>

scripts/record-demo.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Record asciinema demo casts at a consistent terminal size.
+# Usage: ./scripts/record-demo.sh [cast-name]
+# Without arguments, records all four standard casts.
+# With an argument, records just that one (import, pivot, drill, formulas).
+
+CAST_DIR="docs/casts"
+COLS=100
+ROWS=30
+IDLE_CAP=2
+
+mkdir -p "$CAST_DIR"
+
+record() {
+    local name="$1"
+    local outfile="$CAST_DIR/${name}.cast"
+    echo "Recording $name → $outfile (${COLS}x${ROWS}, idle cap ${IDLE_CAP}s)"
+    echo "Press Ctrl-D or type 'exit' when done."
+    COLUMNS=$COLS LINES=$ROWS asciinema rec -i "$IDLE_CAP" --cols "$COLS" --rows "$ROWS" "$outfile"
+    echo "Saved $outfile"
+}
+
+if [ $# -gt 0 ]; then
+    record "$1"
+else
+    for name in import pivot drill formulas; do
+        record "$name"
+        echo ""
+    done
+fi