Add unicode math shorthand for $...$ regions in labels

[gabo515]

Summary

This is the unicode shorthand half of #88, split out per the review discussion there — the typst math layer and typst backend stay parked on the other branch for later.

Any label (title, axis labels, TextPlot markdown bodies) may embed $...$ math written in LaTeX-ish syntax. Math regions are lowered to inline Unicode by every backend, including the terminal:

Input	Output
$\sigma^2$	σ²
$x_i$	xᵢ
$a \leq b \cdot c$	a ≤ b · c
$\frac{a+b}{c}$	(a+b)/c
$\sqrt{x^2+y^2}$	√(x²+y²)
$\sum_{i=1}^{n} x_i$	∑ᵢ₌₁ⁿ xᵢ

Zero dependencies, no feature flag, always on. No Cargo.toml changes at all.

Design notes

• Inline only, by design. Fractions and radicals lower to a/b and √(…), never stacked 2-D layout — the output is plain text that flows anywhere a label can go (rotated y-axis titles, terminal character grids, markdown bodies).
• All-or-nothing super/subscripts. x^{2n} → x²ⁿ, but if any character in the group lacks a Unicode form (q, most capitals), the whole group falls back to a clean x^(2q) — never a half-substituted mix.
• Never leaks source. The lowering never emits a stray \ or $. Literal dollars are written \$; an unclosed $ is left untouched as ordinary text.
• One module, four call sites. src/render/math.rs holds detection (contains_math), segmentation, the command table, and to_unicode. The SVG, raster (PNG/PDF), and terminal Text arms lower at draw time; markdown TextSpans are lowered once after markup parsing, so **bold** $\sigma^2$ works in TextPlot bodies.

Supported syntax

Greek letters (incl. \varepsilon/\varphi variants), operators/relations/arrows (\pm \times \cdot \leq \neq \approx \in \partial \nabla \infty \to \degree …), \frac{a}{b}, \sqrt{x} / \sqrt[3]{x} (→ ³√x), and ^/_ scripts with {...} groups or braceless \command operands (x^\alpha → x^(α)).

Testing

• 21 unit tests in render::math — substitution, fallbacks, escaped/unclosed dollars, nested chains (quadratic formula end-to-end), no-leak guarantees.
• tests/math_lookup.rs — per-backend integration: terminal, SVG, markdown TextPlot body, escaped-dollar literal.
• 7 smoke tests across plot types (scatter/line/bar) and label slots (title, x/y labels, rotated y-label).
• scripts/terminal_plots.sh — math labels entry for visual terminal inspection.
• cargo ci-test (99 suites), cargo ci-clippy, and the full smoke suite (190 checks) all pass.

Docs

• New reference page Reference → Math in Labels with the substitution table, supported-syntax list, CLI usage, and generated example SVGs (examples/math.rs, committed under docs/src/assets/math/).
• Crate-level docs section in lib.rs; CHANGELOG entry.

🤖 Generated with Claude Code

[Psy-Fer]

This is great.
Few little things I'll change but after I merge it.

Thanks
James