Add \dims macro for dimension annotations (#79)

[dem-extra1]

Adds a \dims macro for annotating expressions with their dimensions, per #79.

Usage

\dims{\mX}{n \times p}

renders as

$$\dims{\mX}{n \times p} \eqdef \underbrace{\mX}_{n \times p}$$

so instead of writing the full \underbrace form you write the annotation semantically.

Definition

\providecommand{\dims}[2]{\ub{#1}_{#2}}

placed alongside the existing \ub / \ubf underbrace shorthands.

Notes

• Using \dims avoids clobbering the standard LaTeX/MathJax \dim math operator (e.g. $\dim(V)$), allowing clean definition via \providecommand.
• Added the required interpretations.tsv entry (\dims).

Verification

• Syntax & comment check (macros.qmd has no comments)
• TSV interpretation mapping check (macros-table.qmd parsing requirements met)
• CI preview build

Closes #79.

[github-actions]

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