Incubate a standalone authored font source library

[kostyafarber]

Problem

Shift's format readers and writers are coupled to editor-specific models, native paths, and product workflows. Other Rust, TypeScript, browser, and Node consumers cannot reuse the authored-source work without importing Shift or reconstructing format behavior independently.

Expected outcome

A standalone fontsrc library reads and writes authored font sources through format-native APIs and host-provided I/O boundaries. It incubates in the monorepo against Shift's fixtures and adapters, then moves to the public shift-editor/fontsrc repository without carrying any shift-* dependency.

Acceptance criteria

• The Rust library has no direct or transitive dependency on a shift-* crate.
• UFO, Designspace, and Glyphs inputs have format-native read APIs that do not require native filesystem paths.
• Supported writes preserve format-specific data, including pass-through lib content, and have corpus-backed round-trip coverage.
• Browser and Node consumers have typed TypeScript APIs backed by the same Rust format implementation.
• Multi-file projects can be loaded through host-provided sources without Electron, N-API, persistence, or a server round trip.
• Shift consumes the library through a separate adapter instead of exposing editor identity in fontsrc.
• The incubated library is transferred to shift-editor/fontsrc with documentation and reproducible release configuration before its first public release.

Boundaries

Compiled OpenType previewing, Shift editor sessions, workspace persistence, and the MemoryFontSource adapter are integrations built on or alongside this library, not part of its format-native core.

Related to #399 and #404.