epic: implement Stage 3 — advanced features

[edloidas]

Tracking issue for all Stage 3 features from the PRD. These bring the library to Roll20 parity with grouped rolls, variable injection, rich output, and metadata modifiers.

Features:

• Variable injection — 1d20+@str, 1d20+@{modifier}
• Grouped rolls — {1d8, 1d10}kh1
• Sorting modifiers — 4d6s, 4d6sd
• Critical/fumble thresholds — 1d20cs>19, 1d20cf<2
• Rich JSON parts output — structured breakdown per sub-expression

Each feature will be broken into its own issue when implementation begins.

Rationale

Stage 2 covers the most widely-used system notations. Stage 3 adds the advanced features needed for full Roll20 parity: macro variables, grouped roll operations, visual-only sort/threshold metadata, and structured roll breakdowns for rich UI rendering.

_{Drafted with AI assistance}