feat: preserve the authored autolink form on save

[Azganoth]

Summary

A URL authored as a GFM autolink literal is now written back as one, and one authored with angle brackets keeps them. Editor rendering is unchanged either way.

• The two forms parse into the same link node, so the authored form is recorded on the link mark at parse time, where the source span still distinguishes them, and honored at serialize time.
• A bare literal serializes under its own Markdown type. That keeps the default link handler reachable for every other link rather than reimplementing the angle-bracket and inline forms, and the bare handler delegates to it whenever the literal is not safe to write.
• A bare literal is only written where GFM reads it back: its text must still spell its target, and its neighbouring characters must neither hide the link nor join its target. Otherwise the link falls back to the angle-bracket or inline form, so a saved target cannot change.
• The form travels through the DOM as data-bare-autolink, because an in-editor copy and paste round-trips through the DOM rather than through Markdown.
• Link label bounds in source projection now start at the first child's offset instead of one past the link's, which was correct only for a form with an opening marker.

Related Issue

Closes #240

Verification

Markdown compatibility fixtures cover both forms surviving open and save, on their own and mixed in one paragraph, including the www. and address literals whose targets GFM rewrites, trailing punctuation, and a balanced and an unmatched parenthesis in a path. Two further fixtures pin the fallbacks: an edit that makes the text stop spelling the target writes the inline form, and an edit that closes text in on the link writes the angle-bracket form. Source-projection tests assert that both forms project as their authored source and commit unchanged, and native clipboard tests carry both forms through a copy and a paste.

Round-tripped corpus/gfm/autolinks.md against this branch and against main. Every autolink line in it is now written back unchanged; on main each one was rewritten, www.example.com/path into an inline link and addresses into angle-bracket autolinks. The remaining differences in that file are unrelated pre-existing normalization, identical on both branches.

Not verified: the desktop E2E suite and manual verification in the Tauri application. The change is serialization and projected document text, both observable in the automated DOM tests.

Notes

The neighbour check uses character sets measured against the GFM version this repository pins rather than derived from the specification text. If those drift, the failure mode is a bare URL written as <https://…>, which is the behavior before this change, not a corrupted target.

Preserving the form across the clipboard goes past the issue's acceptance criteria, which cover open, save, and projection. Without it a bare URL copied and pasted inside the editor would gain angle brackets on the next save, which reads as the same churn the issue removes.