clarify thm-deriv-lincom

CLAUDE.md

@@ -51,6 +51,10 @@ Before committing any `.qmd`, `.R`, or config file change:
 - Key macros: `\E{Y|X=x}`, `\ba`/`\ea`, `\tp{v}`, `\b`, `\g`, `\a`, `\devn(...)`, `\erf{...}`
 - Include every intermediate step in derivations — do not skip steps
 - Color coding: `\red{...}` for focal/extra terms, `\blue{...}` for shared terms
+- **Matrix dimensions**: always verify dimension compatibility for every matrix expression -- dimensions of each operand must be consistent with the operation
+- **Annotate matrix dimensions with underbraces** in display math: use `\underbrace{M}_{m \times n}` for each matrix or vector
+- **Zero matrices**: never write bare `\mathbf{0}` in a matrix equation -- subscript dimensions: `\mathbf{0}_{m \times n}`
+- **Jacobian**: `\deriv{\vb} \vx` where both are p-vectors produces a p × p Jacobian matrix (not a vector)
 - Ratios vs. factors:
   - Use the generic `\ratio`/`\ratiof` macro when a ratio's inputs are the **quantities themselves** (the odds, hazards, rates, etc.) — e.g. `\ratio(\odds_1, \odds_2)`, **not** `\ror(\odds_1, \odds_2)` — because the type of ratio is clear from the inputs.
   - Use the type-subscripted ratio macros (`\ror` for odds ratios, `\hazratio`/`\hr` for hazard ratios, `\rateratio`, `\riskratio`, `\prevratio`, `\cuhazratio`, …) only when the inputs are **covariate patterns** (e.g. `\ror(\vx,\vxs)`, `\hr(t\mid\vx:\vxs)`), where the subscript is needed to say which kind of ratio it is.