feat: support \mathrel and \joinrel

[Happypig375]

Summary

• add structural MathRel and JoinRel atoms with canonical parse/serialize support
• apply relation-class outer spacing while preserving enclosed layout and scripts
• implement active-style -3mu join advance without turning it into visible space
• measure recursive ink separately from logical advance so boundary and wrapped joins do not clip
• keep ink bookkeeping internal and align direct/aligned drawing to the same bounds

Stack

This is intentionally based on #276 (fix/issue-182-control-word-boundary) because the new control words rely on that parser-boundary fix. The #310 change itself is commit 9826ac14.

Validation

• Core parser/typesetter focus: 351/351
• prior full Core run on this implementation: 1543/1543
• focused existing Rendering measure/forms: 11/11
• inherited SkiaSharp join-rel raster cases: 4/4
• inherited Avalonia join-rel raster cases: 4/4
• direct tight/boundary Skia cases: 8/8
• Core and Rendering Release builds: 0 warnings/errors
• git diff --check

Closes #310