AJAX navigation: detect SX-Request/HX-Request headers and return just
the #main-panel fragment instead of the full page shell. Fixes layout
break where header and content appeared side-by-side after navigation.
New navigation test suite (tests/playwright/navigation.spec.js):
- layout stays vertical after clicking nav link
- content updates after navigation
- no raw SX component calls visible after navigation
- header island survives navigation
- full page width is used (no side-by-side split)
All 5 tests pass. 14 total Playwright tests pass.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
giles
4294ee3d94