Modernize LaTeX template into Nature-styled manuscript

[medewitt]

Summary

Modernizes the ptmp latex template into a Nature-styled single-column manuscript and makes the emitted project build on the first run.

Changes

• src/templates/main.tex — rewritten from the bare article skeleton into a modern manuscript: natbib with [numbers,sort&compress,super] + \bibliographystyle{unsrtnat} for sorted, superscript numeric citations; modern preamble (microtype, siunitx, booktabs, caption, cleveref + hyperref loaded last, parskip, xcolor); Nature manuscript structure (abstract, Introduction/Results/Discussion, unnumbered Methods, and back matter for data availability, acknowledgements, author contributions, competing interests).
• src/components.rs — the latex command now prompts for email (correspondence line) and ships references.bib alongside main.tex.
• src/templates/references.bib — removed the literal {{ title }} placeholder (was written un-rendered) and added a sample entry so the first build resolves.
• src/render.rs — added tests rendering MAIN_TEX with full and empty-optional contexts; these caught two LaTeX/Tera collisions (\graphicspath{{figures/}} and \author{%), both fixed.
• README.md — updated the ptmp latex description.

Verification

• cargo fmt, cargo clippy (clean), cargo test (26 passed).
• Rendered output compiled end to end with latexmk: PDF builds, BibTeX resolves, references render with DOI, zero undefined citations on the final pass.

Follow-up

• Add sn-jnl.cls (official Springer Nature) LaTeX variant #3 tracks adding an official Springer Nature sn-jnl.cls variant.