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>

scripts/validate-layouts.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+# Validate all EurKEY keylayout files against the v1.3 reference spec.
+# Exit code 0 if all pass, 1 if any unexpected mismatches.
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+exec python3 "${SCRIPT_DIR}/validate_layouts.py" "$@"