feat: add LT_HEX(0xRRGGBB) color-packing macro

[rizukirr]

Summary

Add a public LT_HEX(0xRRGGBB) macro that packs a hex color into bits 0–23 of an lt_attr, complementing the existing LT_RGB(r,g,b).

Changes

• Add LT_HEX(rgb) macro in include/libterm/libterm.h beside LT_RGB, masking to 0xFFFFFFu so attribute/LT_HI_BLACK high bits can never leak. Add tests/test_color_macros.c (compile-time _Static_assert checks for packing parity, masking, and default/black semantics) and register it in CTest.
• Note LT_HEX in the ROADMAP color row and add a CHANGELOG [Unreleased] entry.

Testing

• cmake --build build → exit 0
• ctest --test-dir build → 100% tests passed, 0 failed out of 38 (incl. test_color_macros)
• clang-format --dry-run --Werror (v22.1.6) → clean

Notes

• LT_HEX(0xRRGGBB) packs the same bits as LT_RGB(0xRR, 0xGG, 0xBB); a packed 0 means terminal-default (OR in LT_HI_BLACK for real black), matching LT_RGB semantics.
• No termbox2-compat alias (upstream has none); no changes to SGR emission.