Add bus blocks

[milanofthe]

Adds Bus Creator and Bus Selector blocks to group signals into one wire.

• Bus Creator bundles its inputs into a bus, Bus Selector picks signals out of it. Both live in a new library category Buses and are drawn as trapezoids; wires carrying a bus are twice as thick, bus ports have a heavier outline.
• Buses exist only in the editor. Before code generation they are resolved into direct wiring, including buses passing through subsystem ports: src/lib/bus/expand.ts for the editor, pathview/buses.py for pathview convert. tests/fixtures/bus_expansion.json is shared by both test suites.
• Signal names are the labels of the wires into a Bus Creator and can be edited inline at the port. Selector signals are picked in the properties dialog or inline at the port. Renaming a signal asks whether selectors using it, also in subsystems, should follow.
• Port handles, port labels and the +/- port controls moved from BaseNode into a shared NodePorts component; the connection label editor became a shared InlineInput.
• pvm spec: section 4.4 Buses.

Models without bus blocks generate the same code as before (checked on all examples).

Checks: npm test, pytest tests/, svelte-check, vite build.