feat(fontsrc): add shared project file source

[kostyafarber]

Summary

• expose fontsrc::FileSource as the shared host-owned project tree for native, in-memory, archive, browser, and remote inputs
• add source-backed Designspace and Glyphs entry points with normalized relative-path enforcement
• load native .glyphspackage directories through the same entry point when the source exposes a filesystem root
• return an explicit unsupported error for in-memory packages rather than duplicating glyphs-reader's private package parser

This is stacked on #436. Browser .glyphspackage parsing remains an upstream glyphs-reader capability gap; .glyphs, Designspace XML, referenced UFOs, and UFO directories now share the approved project-source boundary.

Issue

Refs #431

Testing

• cargo test -p fontsrc
• cargo clippy -p fontsrc --all-targets -- -D warnings
• RUSTFLAGS='-D warnings --cfg getrandom_backend="wasm_js"' cargo check --target wasm32-unknown-unknown -p fontsrc
• RUSTDOCFLAGS='-D rustdoc::broken_intra_doc_links' cargo doc -p fontsrc --no-deps --document-private-items
• verified cargo tree -p fontsrc --edges normal --prefix none contains no shift-* crate
• cargo fmt --all -- --check
• git diff --check
• python3 scripts/context-drift-check.py reports only 11 existing freshness warnings outside this change

Desktop E2E was not run because this changes only the standalone Rust source boundary.