Phase 4.4: bundled interface typeface

[danielmrdev]

Closes #30

Bundles iA Writer Quattro as the Interface typeface for non-editor chrome (Document selector, feedback, search).

Deviation from spec

The spec said static Regular+Bold; both static S faces ship OS/2 usWeightClass 608 → fontdb normalizes to weight 400 for both, so NORMAL and BOLD queries resolve to the same face and interface bold would not render. Verified empirically. Bundle instead the variable font iAWriterQuattroV.ttf (family iA Writer Quattro V, wght 400–700): shaping test proves bold ≠ normal metrics. OFL license bundled at assets/fonts/iA-Writer-Quattro-LICENSE.md.

Changes

• src/fonts.rs: INTERFACE_TYPEFACE_FAMILY, register_bundled_interface_typeface() (include_bytes, non-fatal), 2 tests
• src/main.rs: register at startup (normal + smoke paths)
• src/editor.rs + src/shell.rs: interface_label helper applies family to selector/feedback/search surfaces
• assets/fonts/: iAWriterQuattroV.ttf + license
• Docs: research note deviation section, phase-4 spec line, CHANGELOG

Note

Branch carries #29 FontDelivery commits (d2994b2, 968ff7c) ahead of main; they land with this PR (merge commit keeps history, as with #28 via #34).

Verification

• 197 tests pass, clippy -D warnings clean, fmt clean
• Linux shaping evidence in research note