add validation infrastructure, fix "6" key bug in v1.3

- add keylayout XML parser (parse_keylayout.py) that extracts all key
  mappings, dead key compositions, modifier layers from .keylayout files
- add validation script (validate_layouts.py) that compares layouts against
  v1.3 reference with per-version exception support
- fix action id="6" outputting "p" instead of "6" in v1.3
- generate reference JSON for all 4 layout versions
- document known intentional differences: v1.2 (no ¬ dead key, § vs ẞ),
  v1.4 (ẞ on §/` caps, extra ¬ composition)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

EurKey-macOS.bundle/Contents/Resources/EurKEY v1.3.keylayout

@@ -1123,7 +1123,7 @@
             <when state="dead: Ω" output="⁵"/>
         </action>
         <action id="6">
-            <when state="none" output="p"/>
+            <when state="none" output="6"/>
             <when state="dead: ¬" output="¾"/>
             <when state="dead: Ω" output="⁶"/>
         </action>