v0.3.1 — Geometry, export and reliability fixes (alpha)

This alpha patch fixes bugs in PCB geometry, manufacturing exports, installation and the MCP transport. The server still provides 28 tools.

• Footprint moves now preserve polygon arcs, holes and all zone contours/fills, and reject child coordinates outside KiCad's geometry range.
• Stackup thickness includes every dielectric sublayer. Failed layer restoration is reported explicitly.
• Project-relative library paths remain correct when they contain variables or dollar signs. Empty exports and malformed DRC findings are rejected.
• BOM update acknowledgments must preserve geometry and unrelated footprint data.
• Plugin updates and PCM builds preserve the previous working files if preparation fails, avoid overwriting unrelated hard-linked files, and remove stale Python caches.
• MCP arguments keep their declared boolean/number types. Local HTTP sessions handle malformed records, invalid tokens, localhost headers and conflicting read-only restarts correctly.

401 automated tests pass, including 76 added regression cases. CI passes on Python 3.10, 3.12 and 3.13.

Install the ZIP through KiCad's Plugin and Content Manager using Install from File, or update a manual installation with scripts/install_plugin.py --overwrite.

DRC and manufacturing tools continue to use the last saved PCB and project settings, without implicitly saving the original. This is an alpha release: validation in a real KiCad GUI and against a real kicad-cli remains pending.