Enforce bus wiring rules and highlight connectable ports

[milanofthe]

• A bus may only be connected to a Bus Creator, a Bus Selector or a subsystem port, and a Bus Selector only takes a bus. Wires breaking these rules are rejected while connecting (isValidConnection) and existing ones are drawn dashed in the error color.
• While a wire is dragged, the ports that can take it are outlined in their block color: free inputs when dragging from an output, outputs when dragging from an input, both within the bus rules.
• The rule is busWiringProblem in src/lib/bus/expand.ts, with tests including subsystem ports. The bus analysis is computed once per graph change and reused during drags.
• pvm spec 4.4 notes that editors should not create such wiring.

Checks: npm test, svelte-check.