Support spacer children in headers

[Draginol]

Summary

• Allow direct spacer children under header for left/right toolbar alignment.
• Keep parser placement errors and documentation consistent with the expanded grammar.
• Add parser and layout regression coverage.

Root cause

Headers used the general container-child parser, while the renderer already handled direct spacer children as horizontal toolbar bands.

Validation

• Targeted parser and renderer tests: 106 passing.
• npm run typecheck: passing.