Derive layout PCB path from .kicad_pro file

[akhilles]

Previously, we tried to tolerative missing .kicad_pro file by also looking for a unique .kicad_pcb file but this is unnecessary complexity. We can safely assume a .kicad_pro file is present/necessary.

---

Note

Medium Risk
Changes the file-discovery contract for layout tooling; workflows/layout dirs that previously relied on a lone .kicad_pcb without a .kicad_pro will now be treated as missing layout and skip/fail related tasks.

Overview
Layout discovery is tightened to only accept a single top-level .kicad_pro in a layout directory, and the corresponding .kicad_pcb path is now always derived from that project file (avoiding false ambiguity from autosave/extra .kicad_pcb files).

This behavior change is applied across the Rust pcb-layout utilities and the Python lens KiCad adapter, and pcb release/layout detection messaging is updated to reflect the new requirement; the changelog records the fix.

^{Written by Cursor Bugbot for commit d70dec8. This will update automatically on new commits. Configure here.}