Backport Excel theme tint color matching to v2

[developer0hye]

Problem

The office2pdf v2 dependency resolves positive theme tints through a floating-point 255-step HLS space. Native Excel uses integer 240-step HLS behavior, which makes DAB6BA at tint 0.7999 resolve to F8EFF0 rather than F8F0F1.

Fix

Backport the theme tint correction from MathNya#360 while retaining the v2 public API:

• use the 240-step integer HLS conversion
• recover near-whole-percent encoded tint values within one signed-short step
• preserve arbitrary tint precision outside that threshold
• truncate adjusted luminance before the reverse conversion
• update native fixture expectations and add direct regression tests

Testing

• rustfmt +nightly --edition 2018 --check src/helper/color.rs tests/integration_test.rs
• cargo clippy -- -D warnings
• cargo test (41 unit, 97 integration)

Related: MathNya#360
Related: developer0hye/office2pdf#1394