Add scripts/stage-plugins.sh and run-sample.sh for running the engine locally

Contracts and Implementation build into two separate bin/ folders, so
there's nowhere valid to point --plugins at without a staging step first
— every manual verification this session has done that staging by hand.
stage-plugins.sh does it for every plugin in one pass; run-sample.sh
stages then runs samples/WindowDemo windowed, forwarding extra args to
`engine run` (e.g. --screenshot).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N1qPfzq8TDCUMFMV3UwV5N

scripts/run-sample.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+# Stages every plugin, then runs samples/WindowDemo windowed. Extra args
+# are forwarded to `engine run` — e.g. ./run-sample.sh --screenshot out.png
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+CONFIG="${CONFIGURATION:-Debug}"
+STAGE_DIR="$REPO_ROOT/.stage/plugins"
+
+"$REPO_ROOT/scripts/stage-plugins.sh" "$STAGE_DIR"
+
+cd "$REPO_ROOT/samples/WindowDemo"
+exec dotnet "$REPO_ROOT/src/Engine.Host/bin/$CONFIG/net9.0/Engine.Host.dll" run --windowed \
+    --plugins "$STAGE_DIR" --project project.json --scene scene.json "$@"