Add support for tracking (letter-spacing) REDUX

[JimBobSquarePants]

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Description

This is a rework of #492 (thanks @Socolin) that introduces a Tracking property on TextOptions, allowing uniform adjustment of spacing between characters, measured in em units.

To support this correctly, fixes were required in SpanGraphemeEnumerator to handle Indic conjuncts involving virama correctly. Specifically, a GB9c-style special case was added so sequences such as consonant + virama + consonant are treated as a single grapheme cluster rather than being split into a “dead consonant” followed by a new grapheme. This ensures tracking is applied at the correct cluster boundary instead of being injected inside a shaping unit.

Additional fixes were required in vertical layout paths to ensure grapheme boundaries are respected consistently when applying tracking. This includes handling graphemes composed of multiple codepoints correctly in both vertical and vertical-mixed rendering modes, preventing incorrect splitting, misalignment, or over-accumulation when glyphs are rotated or laid out in vertical flow.

Example rendered layout is included in the reference tests.

[Socolin]

Looks good ! Thanks !