Beta: Glass 0.2.0 document facade editor

[cowboycodr]

Summary

• Adds a structured document model and Markdown codec behind the editor.
• Switches buffer save, copy, link navigation, checklist toggles, and table insertion paths to document-backed Markdown serialization.
• Keeps the existing terminal UX while making Markdown rows facade-rendered, including active rows and tables.
• Adds :table / :table 3x4 plus table-cell Tab navigation.
• Splits navigation and actions into friendlier entry points: Cmd+P opens files, Cmd+Shift+P opens the command palette, / searches, and : remains the terse Vim command line.

Validation

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

This is a beta PR for the 0.2.0 document facade architecture.

[cowboycodr]

Superseded by the cleaner stacked 0.2 PRs: #3 for the document facade foundation and #4 for the command/file navigation split.