[codex] v0.2.0 document facade editor

[cowboycodr]

Summary

• Introduces the structured document layer and Markdown codec for Glass v0.2.0.
• Moves editing state toward document-backed buffers with canonical Markdown save/copy serialization.
• Preserves current terminal editor UX while facade-rendering Markdown rows and tables.
• Adds document-native table insertion and table-cell navigation support.

v0.2.0 Stack

This is the first PR in the Glass v0.2.0 stack. It contains the document facade foundation.

Follow-up stack PR:

• [codex] v0.2.0 command surfaces #4 splits file navigation and the command palette on top of this foundation.

Validation

• cargo fmt --all -- --check
• cargo test --locked
• cargo build --release --locked
• git diff --check