LTML parity: implement absolute/relative positioning and transforms

[rowland]

Make the documented LTML positioning and transform behaviors real and testable.

Scope:

• implement functional absolute and relative layout modes
• support positioned widgets via top/right/bottom/left semantics
• add rotate, origin_x, origin_y, shift, and z_index-related behavior
• define ordering and layout interactions clearly
• add rendering tests for positioned, shifted, and rotated widgets

Dependencies:

• PDF transform support

[rowland]

Completed by commits 85bbc6a (positioned layout semantics), 6d3457a (widget transforms), and bbb3546 (z_index support). LTML now supports absolute/relative positioning, top/right/bottom/left semantics, rotate/origin_x/origin_y/shift/z_index behavior, documented ordering rules, and rendering-oriented tests/samples.