Sync misc/qet-mcp with the extended scripting API

Brings misc/qet-mcp up to date with the scripting API additions in
this branch: qet_edit gains ops for tables, PLC master IO tables and
PLC-slave linking, manual conductor segment routing, polygon and path
shapes, PDF page import, and project-wide search & replace; a new
qet_continuity tool exposes the electrical continuity/ERC-style checks.

Adds the test suite that did not exist here before (150 tests: unit
validation, JSON-RPC protocol, and Integration/PlcIntegration/
CorpusIntegration runs against a built binary) plus the two minimal
PLC fixture .elmt files it needs (no shipped element has masterType/
slaveType "plc" to test against).

Also removes a __pycache__/*.pyc that had been committed by mistake,
and ignores __pycache__/*.pyc going forward.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -1,5 +1,7 @@
 *.snap
 .flatpak-builder
+__pycache__/
+*.pyc
 # Qt build output
 *.user
 # doxygen Doxyfile output