fix(pdf): reject unsafe font encodings

[McanKul]

Summary

• classify subset simple fonts as unsupported with SUBSET_FONT
• classify custom, Differences-based, or unresolved simple-font encodings as unsupported with CUSTOM_ENCODING
• retain support for the named standard encodings already handled by the prototype
• keep this read-only: no UI, mutation, save path, release, or main-branch changes

Verification

• cargo test --lib source_content — 65 passed
• cargo test --lib — 322 passed
• cargo clippy --lib --tests — no new warnings attributable to this change

Refs #33