Surface unknown node types; escape rule interpolations

[jpcamara]

A custom editor node with no render rule doesn't break to_html — it quietly degrades: an unknown container renders its children without its own markup, and a node whose content lives only in its attributes renders nothing. Nothing reported that. This PR makes the degradation visible and hardens the escaping story around rules.

unknown_types

Y::Lexical#unknown_types / Y::ProseMirror#unknown_types (inherited by Y::Lexxy and Y::Tiptap): the node types the document contains whose handled is nil in node_types — exactly the set to_html degrades. It's the checklist companion to the existing discovery aid: render a real document from your editor in a test and assert the list is empty. Returns [] for a missing or wrong-shaped root, where to_html returns nil and nothing degrades.

Escaping

• Y::Lexxy's list-item rule interpolated the stored __checked and __value attributes raw. Stored attributes are collaborator input — a crafted value could break out of the attribute position. Both now go through RenderRules.escape_attr. Legitimate editor values (booleans, integers) render byte-identically, so the parity fixtures are unaffected.
• The renderer crates make escape_text / escape_attr public for splice callers, and both crate READMEs' callback examples now escape what they interpolate instead of demonstrating the unsafe pattern in a comment. lexical-yjs-html 0.1.1 → 0.1.2, prosemirror-yjs-html 0.1.2 → 0.1.3.

Docs

The crate READMEs and the root README's rules section state the degradation contract (unknown containers unwrap; attribute-only nodes render nothing; on the ProseMirror side, unknown marks render their text unformatted) and point at unknown_types.

Tests: unknown_types across all four renderers plus the rule-covered and missing-root cases; a hostile __checked value renders escaped. Crate doctests cover the updated README examples.