feat: add reusable LaTeX component library

[szmyty]

Summary

• extract shared color, typography, layout, figure, table, code, callout, and metadata presentation primitives into a modular LaTeX component layer
• refactor the research Pandoc template into a profile-level composition that consumes the shared layer and exposes semantic token/font overrides
• inject absolute style and optional local-font roots through the existing PDF strategy without overriding profile configuration
• add a redistribution-safe component showcase fixture and architecture/extension documentation

Verification

• cargo check --workspace --locked
• cargo clippy --workspace --lib --bins --all-features --locked -- -D warnings -A clippy::too_many_arguments
• renderflow spec validate --config "tests/fixtures/latex-components/renderflow.yaml"
• Pandoc expansion of the synthetic showcase through templates/research/research.tex
• two-pass XeLaTeX artifact smoke: valid 2-page A4 PDF
• missing local-font smoke: warning emitted and deterministic Latin Modern fallback compiled
• git diff --check

Tectonic and MkDocs are not installed in the local environment. The full test suite was intentionally skipped for this sprint, per the current speed-run policy.

Closes #381