Use ports for PhasorDynamics signal connectivity

[lukelowry]

Description

This PR updates PhasorDynamics component wiring so signal nodes are associated with input/output ports instead of internal/external variable enums.

Closes #407

@PhilipFackler, what are your thoughts? Managed to do it with no *.case.json changes, which lets us do the INPUT_FORMAT changes and documentation with a few changes in code once we get to it

Proposed changes

• Split component connection data into terminals, input ports, and output ports.
• Keep the existing flat JSON "ports" format, but parse entries into the new internal maps.
• Update ComponentSignals, SystemModel, components, examples, and tests to use port enums for signal wiring.
• Remove the old variable-enum signal helpers.

Checklist

• All tests pass for the focused coverage.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.
• I have updated CHANGELOG.md to reflect the changes in this PR.

Further comments

I organized the commits so we can roll back if the scope is too big, but it was mechanical and straightforward after the main changes were in.