feat: add renderLatexPDF subprocess renderer

[rohitm-1729]

Summary

• Adds src/lib/pdf/render-latex-pdf.ts exporting renderLatexPDF(tex: string): Promise<{ buffer: Buffer; pages: number }>
• Uses execFile (not exec) for security — avoids shell injection
• Writes LaTeX source to a temp directory, runs pdflatex -interaction=nonstopmode, reads the output PDF, then cleans up the temp dir in a finally block
• Page count is derived from /Type /Page markers in the raw PDF binary; defaults to 1 if none found

Test plan

• Unit tests cover happy path (buffer returned, page count returned)
• Verifies pdflatex is called with -interaction=nonstopmode and -output-directory flags
• Verifies temp directory is cleaned up even when pdflatex fails
• Verifies temp directory is cleaned up even when readFile fails
• Verifies multi-page detection and default-to-1 fallback
• All 7 tests pass (npm test)

Closes #34

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