Fix floating window frame chrome

[elithrar]

Restores the continuous frame rail between floating Window title tabs and their bodies.

• Exposes the 3px beveled frame rail without returning border ownership to the body.
• Anchors active and inactive shadows to the visible frame surface instead of the transparent wrapper.
• Preserves stacked, automatic, and collapsed chrome behavior across responsive widths.
• Documents the floating and stacked title treatment with an inactive floating fixture.
• Adds CSS ownership and rendered geometry regression coverage.