Add warm-engine mechanisms for autopdf

[BuddhiLW]

✨ Feature Request: Warm-Engine (Persistent LaTeX Worker)

Summary

Add a mode to AutoPDF in which the LaTeX engine (pdflatex, xelatex, or lualatex) remains alive and pre-loaded across multiple document builds.
This persistent (“warm”) mode avoids re-initializing packages, fonts, and macros for every render — drastically improving throughput and latency for API or batch workloads.

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Motivation

Currently, each AutoPDF run spawns a new LaTeX process that performs full initialization (format load, font cache, macro expansion, hyphenation setup).
This is acceptable for ad-hoc jobs but inefficient in backend scenarios that generate many similar PDFs.

A persistent engine can:

• Eliminate redundant startup overhead.
• Reduce per-document build time from seconds to milliseconds.
• Improve scalability for services like cartas-backend, obito-backend, or other AutoPDF clients.

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Goals

• Introduce an optional warm mode: --warm, --daemon, or via config.
• Support worker pools: one or more long-lived LaTeX subprocesses.
• Allow .fmt pre-loading for faster engine startup.
• Maintain backward compatibility (cold mode remains default).
• Provide configurable knobs: pool size, idle timeout, jobs per worker.

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Scope & Deliverables

1. Worker Manager

• New module (e.g., internal/autopdf/worker/) managing LaTeX subprocesses.
• Worker struct wraps a persistent LaTeX process with stdin/stdout handles.
• Each worker:
  • Accepts .tex jobs via stdin or temp files.
  • Emits logs and output PDFs.
  • Can restart automatically on error or timeout.

2. Persistent Engine Invocation

• Launch engine with -fmt=<custom> (if provided) for faster startup.
• Keep process open between builds.
• Clean job isolation (per-job workdir, *.aux cleanup).
• Graceful restart on crash.

3. CLI / Config Additions

autopdf build --warm \
  --worker-pool-size=4 \
  --worker-idle-timeout=300s \
  --preload-fmt=/path/to/autopdf.fmt \
  template.tex config.yaml

YAML Config Example:

engine:
  warm:
    enabled: true
    poolSize: 2
    idleTimeout: 60   # seconds
    fmtFile: "autopdf.fmt"

4. Precompiled Format Support

• Optional .fmt file created via:

  pdflatex -ini -jobname=autopdf "&pdflatex" \input{autopdf.ltx}

• Allows LaTeX to skip package loading on every run.

5. Error Handling

• Capture engine stdout/stderr and exit codes.
• Restart failed workers automatically.
• Clean shutdown of idle workers.
• Optional job retry/back-off strategy.

6. Testing & Benchmarking

• Benchmark warm vs cold mode latency.
• Verify correctness (no cross-job artifacts).
• Document measurable performance gains.

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Benefits

• 3–5× faster PDF generation under load.
• Predictable latency for web/API workflows.
• Lower CPU usage and I/O overhead.
• Improved competitiveness for high-volume deployments.

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Risks / Considerations

• Persistent workers may accumulate memory; periodic restarts needed.
• Job isolation (e.g., leftover .aux) must be ensured.
• Added complexity — must remain modular and optional.

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Success Criteria

• Warm mode is opt-in and backward compatible.
• Multiple consecutive builds show significant latency reduction.
• Worker pool stable under parallel load.
• Clear documentation and examples.

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Suggested Next Steps

1. Implement Worker and WorkerPool abstractions.
2. Add CLI/config integration.
3. Support .fmt pre-loading.
4. Write integration tests and benchmarks.
5. Update README and examples.