fix(serialization): prevent PostProcess project-load crash

[barrulus]

Loading a project containing a PostProcess connection crashes with Port not found: input: the node declares in, but both model and UI legacy converters normalize saved in links to input. This also affects projects saved by the current application.

Rename the PostProcess input to input to match the existing conversion and update its bundled port documentation. Extend the graph-editor loading regression test to cover both in and input, verify model/UI connections remain consistent without triggering computation, and verify save/reload preserves the connection.

Validation:

• Reproduced the exact exception before the fix with the new legacy-port test and the reporter's original Terrain_v002.hsd.
• Built the application and tests with the locked Nix dependencies; all 3 CTest entries passed, including all 31 graph-editor cases.
• Loaded the original 4096×4096 project and round-tripped its model/UI serialization: all 42 nodes, 52 connections, and PostProcess parameter values were preserved. This checked loading and serialization, without terrain evaluation.
• git diff --check passed.

CI follow-up: install qtserialport alongside the existing Qt modules. The AppImage packager deploys Qt's NMEA positioning plugin, which requires libQt6SerialPort.so.6; the missing library also breaks upstream dev. The updated workflow passes actionlint; AppImage validation is pending the new CI run.

Fixes #775.