test(theme): querya.theme.v1 parser fixtures (#97)

[ZhuchkaTriplesix]

Summary

• Add stable test fixtures for the custom theme parser epic:
  • querya_custom_dark.json / querya_custom_light.json — full themes with distinct values
  • querya_custom_minimal.json — small required-field subset
  • querya_custom_invalid_missing_id.json — missing id
  • querya_custom_invalid_color.json — invalid optional colors
  • querya_custom_jsonc.jsonc — comments and trailing commas

Test plan

• All .json fixtures parse with json.loads
• JSONC fixture follows existing JSONC test pattern (stripped before parse in code)

Depends on #127 (TP-01 docs). Merge #127 first, then re-target this PR to dev if needed.

Closes #97