[codex] Add editable live preview surfaces

[cowboycodr]

Summary

Implements an Obsidian-style editable Live Preview path for the document model editor.

• Adds a shared document surface layer with display text plus source/display mappings.
• Routes active rendering, wrapping, cursor placement, mouse mapping, and surface edits through that layer.
• Reveals editable heading markers and inline syntax near the cursor while keeping list, checklist, and table structure rendered.
• Preserves Markdown shortcut conversion and makes malformed inline delimiter edits degrade to literal text.
• Updates --render full-height calculation to use the same active surface wrapping as the editor.

Validation

• cargo check
• cargo test (172 passed)
• cargo run -- --render --width 90 benchmark.md