feat(latex): add optional Tectonic TikZ rendering (#3369)

* feat(latex): add asynchronous TikZ rendering via Tectonic engine

This commit introduces a high-performance, asynchronous pipeline for rendering
TikZ diagrams into images during LaTeX document conversion.

Key Changes:
- Tectonic Integration (`TectonicEngine`): Compiles `tikzpicture` environments
  into PDFs using Tectonic, auto-downloading the binary if missing. Rasterizes
  the PDF to 300 DPI images.
- Asynchronous Processing: Utilizes a dynamic `ThreadPoolExecutor` (scaled to
  `os.cpu_count() - 1`) to render multiple diagrams concurrently without
  blocking the main document conversion pipeline.
- Preamble Extraction: Dynamically parses the main document's preamble and
  injects it into standalone diagrams to ensure compatibility with complex
  libraries (e.g., `pgfgantt`, `tikz-cd`, `tkz-euclide`).
- Graceful Fallbacks: If Tectonic compilation fails due to LaTeX syntax errors
  or incompatible packages, the engine gracefully falls back to preserving the
  raw TikZ source code as a `CodeMetaField` to prevent data loss.
- CLI Support: Added `--tikz-engine tectonic` option to enable the backend
  configuration.

Resolves pre-commit hooks (MyPy, Ruff linter/formatter).

Signed-off-by: Aditya Sasidhar <arctic@arctic>
Signed-off-by: Aditya Sasidhar <telikicherlaadityasasidhar@gmail.com>

* feat(latex): add optional Tectonic TikZ rendering with isolated dependency staging

  Add opt-in TikZ image rendering for the LaTeX backend using Tectonic,
  while preserving stable fallback behavior when rendering fails.

  What this changes:
  - add optional `tikz_engine="tectonic"` backend support for TikZ diagrams
  - render `tikzpicture` environments asynchronously during LaTeX parsing
  - preserve raw TikZ code as `PictureMeta.code` whenever rendering fails,
    times out, or rasterization cannot complete
  - add Tectonic engine options for:
    - automatic binary download
    - per-diagram timeout
    - shell escape control
  - make shell escape explicit opt-in via CLI/backend config
  - sanitize known pdfTeX-only assignment lines in preambles for better
    Tectonic/XeTeX compatibility
  - restore file-backed relative TikZ compatibility by staging only explicit
    local dependencies (`\input`, `\include`, `\includegraphics`) into the
    temp render directory
  - block dependency path traversal and avoid ambient source-directory search
  - rasterize generated PDFs with locking and crop whitespace from output

  CLI / config updates:
  - add `--tikz-engine` / `-T`
  - add `--no-tikz-engine-download`
  - add `--tikz-engine-timeout`
  - add `--tikz-shell-escape`

  Tests:
  - add focused Tectonic engine tests for download behavior, timeout,
    preamble sanitization, shell escape toggling, dependency staging,
    and path traversal blocking
  - add backend tests for TikZ fallback behavior and file-backed source-root
    handling

Signed-off-by: Aditya Sasidhar <telikicherlaadityasasidhar@gmail.com>

* docs: add documentation for pipeline options

Signed-off-by: Aditya Sasidhar <telikicherlaadityasasidhar@gmail.com>

* fix(latex): keep Tectonic engine within module boundaries

Signed-off-by: Aditya Sasidhar <telikicherlaadityasasidhar@gmail.com>

* fix(latex): fix mypy error by narrowing backend option types

Signed-off-by: Aditya Sasidhar <telikicherlaadityasasidhar@gmail.com>

* revert: drop unintended pyproject and uv lock updates

Signed-off-by: Aditya Sasidhar <telikicherlaadityasasidhar@gmail.com>

* fix(latex): Got rid of the niche latex tikz based tectonic control flags

Signed-off-by: Aditya Sasidhar <telikicherlaadityasasidhar@gmail.com>

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Signed-off-by: Aditya Sasidhar <arctic@arctic>
Signed-off-by: Aditya Sasidhar <telikicherlaadityasasidhar@gmail.com>