Add load_with_options: partial loading and per-source failure reporting

[tomconroy]

Reworked to implement the load_with_options API you sketched in #33, folding in the source-failure reporting this PR originally proposed.

API

pub struct LoadOptions {
    pub load_axes: bool,     // all default to true
    pub load_masters: bool,
    pub load_glyphs: bool,
    pub load_layers: bool,
}

pub struct LoadResult {
    pub font: Font,
    pub source_failures: Vec<SourceLoadFailure>, // { filename, error }
}

pub fn load_with_options(filename: impl Into<PathBuf>, options: &LoadOptions)
    -> Result<LoadResult, BabelfontError>;

load() is now a thin façade over load_with_options and behaves exactly as before (including silently skipping unloadable designspace sources).

Semantics

• load_axes: false — no axes on the returned font.
• load_masters: false — masters are bare records of what the file declares (name, id, location; for designspace sources also filename/style name), and designspace source UFOs are not opened at all. Combined with load_glyphs: false this parses just the designspace XML: your "axes/masters/instances without the sources" case. Implies no layers, since layers belong to masters.
• load_glyphs: false — no glyphs.
• load_layers: false — glyphs are layer-less stubs. For UFO/designspace the names come from the default layer's contents.plist, so no .glif file is parsed anywhere (norad DataRequest::default().layers(false)); lib-derived categories, production names and export flags are kept, codepoints aren't (they live in the glifs). Glyphs-format stubs keep codepoints and kern-group membership.

Implementation notes

• UFO, designspace, and Glyphs convertors skip the ingestion work natively. The remaining formats (fontra, vfj, sfd, rcjk, vfb, ttf, .babelfont) load fully and are filtered afterwards, so every format honours the options with the same result shape; native skipping can be added per-format later without an API change.
• Designspace: the default source is only opened when masters or glyphs are requested. Sources that fail to load land in source_failures instead of vanishing in the filter_map; each failure leaves the font without that master.
• Glyphs format: masters and axes are always converted internally (axis min/max/default interpretation needs the master locations) and the unrequested parts are dropped at the end, so load_masters: false still returns correctly-interpreted axes.

Numbers

Measured against googlefonts/roboto-serif's RobotoSerif.designspace (81 sources, release build) — the case that motivated #33:

options	time	result
full load	5.0 s	77 masters, 1264 glyphs with layers
load_layers: false	460 ms	77 masters with metrics+kerning, 1264 glyph stubs
load_masters/glyphs: false	0.6 ms	81 bare master records, axes, instances

(The remaining 4 of the 81 sources fail on an unrelated norad fontinfo issue — linebender/norad#414 — and are now visible in source_failures rather than silently missing.)

🤖 Generated with Claude Code

[simoncozens]

I'm not sure this dependency is actually used?

[tomconroy]

It's used in the new ufo::converters::default_layer_glyph_names – added so that we can still output the glyph names without any layers:

// full load                              // load_layers: false
{                                         {
  "name": "A",                              "name": "A",
  "category": "Base",                       "category": "Base"
  "codepoints": [65],                     }
  "layers": [ /* 2 layers */ ]
}

[tomconroy]

See #33 (comment)