CORE-008: Font audit and bundled-library proposal

Context

The font compatibility table was compiled from public sources and must be verified by measurement. Fonts are the largest single fidelity risk (R-02), above all Aptos. Microsoft’s font files may be used on the reference machine under its license, but must never leave it; only derived comparison statistics may.

Objective

Measured verification of every substitute in the font compatibility table, corpus font frequencies, and a concrete proposal for the bundled font library and the gap programme.

Scope

In scope

• A font comparison tool in lab/ that compares two font files: advance widths for all mapped code points, vertical metrics (head units per em, hhea, OS/2 typo and Windows ascent/descent, line gap), kerning results for frequent pairs via shaping, and glyph coverage. Output: summary statistics and lists of differing code points with deltas; never full metric tables of Microsoft fonts.
• Run it on the reference machine for every row of the table (including Aptos and its variants, Calibri Light, Cambria Math) against candidate substitutes.
• Font frequency analysis over the public and (aggregate only) private corpora: which families, styles and scripts appear how often.
• A proposal: bundled set (file, version, license, size, SHA-256), mapping table from Microsoft family names to substitutes, fallback chain per script, symbol-font mapping plan, delivery plan (desktop core set versus optional packs; web chunking), and a ranked gap list with recommended action and rough cost.
• Legal-review flags for any font that copies metrics or kerning (for example the early Aptos-compatible project noted in the table).
• Update specs/font-compatibility.md to v1 via a docs pull request.

Out of scope

Implementing the font system (CORE-110); commissioning fonts.

Deliverables

Comparison tool, measurement report, frequency report, docs pull request with spec v1 and the proposal.

Acceptance criteria

• AC-1 Every row of the compatibility table has a measured status (compatible, partially compatible with listed differences, or gap).
• AC-2 The proposal lists total sizes and licenses, and every bundled font is on the ADR-0017/ADR-0010 license list.
• AC-3 The gap list is ranked by measured corpus frequency.
• AC-4 No Microsoft font file or full metric table appears in any repository or artifact (reviewer checks).

Verification

Tool tests on open fonts in CI; reference-machine run logs summarized in the pull request.

Escalate if

Licensing of a candidate font is unclear, or the measurements contradict ADR-0010’s assumptions (for example a “metric-compatible” font that is not).