Add UI-agnostic model layer: typed grouped properties (string/int/float/
bool/enum/color/catalogue item(s)/value list), catalogue registry, port
relations with color schemes and compatibility rules, ports, node/edge
models, and DiagramDocument with change signals and JSON round-trip.
Include a starter node library (source/tank/pump/valve/junction/consumer/
transformer) with matching SVG symbols, plus model+document tests.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
