Add automatic fontsource font detection and downloading

[jonmmease]

Summary

Add automatic font detection and downloading from Fontsource during Vega/Vega-Lite conversions. When auto_fontsource is enabled, vl-convert parses the compiled Vega spec to find referenced fonts, checks them against the local fontdb, and downloads missing fonts from Fontsource before rendering.

Builds on PR #245 (fontsource crate + explicit register_fontsource_font() API). This PR adds automatic detection/download plus configurable missing-font handling.

Motivation

With explicit register_fontsource_font(), users must manually identify which fonts a spec references and pre-install them. This is error-prone, especially for theme-driven fonts or specs with many font references.

This PR separates two independent concerns:

• auto_fontsource (bool) — whether to download missing fonts from Fontsource automatically
• missing_fonts (policy) — what happens when referenced fonts are unavailable: fallback (silent, default), warn, or error

Behavior Matrix

auto_fontsource	missing_fonts	Behavior
false	fallback	Default. No font preprocessing; renderer falls back silently
false	warn	Validate against local fontdb; warn for each missing font. No network.
false	error	Validate against local fontdb; fail if any font is missing. No network.
true	fallback	Download from Fontsource; fall back silently on failures
true	warn	Download from Fontsource; warn on API/catalog/download failures
true	error	Download from Fontsource; fail on API/catalog/download failures

Generic CSS families (serif, sans-serif, etc.) are never flagged as missing. For multi-font CSS strings like "Roboto, Arial, sans-serif", only the first entry is checked/downloaded — this matches Vega's rendering behavior, which tries the first font and falls back to system generics.

Backwards compatible: Both options default to off (auto_fontsource: false, missing_fonts: "fallback"), so existing behavior is unchanged.

Changes

New module: vl-convert-rs/src/extract.rs

• CSS font-family parsing via svgtypes::parse_font_families() (spec-compliant quoting/escaping) with naive-split fallback for non-standard idents
• Vega spec traversal extracting font references from marks, axes, legends, titles, config, and wordcloud text
• resolve_first_fonts() classifies each font string's first entry as Available, NeedsDownload, Generic, or Unavailable
• Unit tests inline (~30 tests) plus integration tests in tests/test_extract.rs

Fontsource client: vl-convert-fontsource/src/client.rs

• is_known_font() / is_known_font_blocking() — lightweight metadata-only check (no font file downloads)
• Checks local cache first, falls back to Fontsource API; returns false on 404

Core library: vl-convert-rs/src/converter.rs

• MissingFontsPolicy enum (Fallback, Warn, Error)
• auto_fontsource and missing_fonts fields on VlConverterConfig
• preprocess_fonts() pipeline: extract → check local fontdb → query Fontsource → report/download
• maybe_compile_vl_with_preprocessed_fonts() for VL→* methods (compiles VL→Vega first to avoid double compilation, preprocesses fonts, renders via Vg→* path)
• maybe_preprocess_vega_fonts() for Vg→* methods
• All 12 rendering methods hooked up (6 Vega→*, 6 Vega-Lite→*)

CLI: vl-convert/src/main.rs

• --auto-fontsource (bool flag)
• --missing-fonts <fallback|warn|error> (value enum)
• Both are global flags available on all subcommands

Python bindings: vl-convert-python/src/lib.rs + vl_convert.pyi

• auto_fontsource and missing_fonts kwargs on configure()
• Reported by get_converter_config()
• Type stubs updated
• pyo3-log integration: Rust log::warn!() / log::error!() messages (missing font warnings, Fontsource API errors) are forwarded to Python's logging module