[codex] Add full-page render debug command

[cowboycodr]

Summary

• Add glass --render / --dump-render as a non-interactive ANSI debug snapshot.
• Default render output to the full document height plus the status bar, with optional --width and --height overrides.
• Document the command and cover parser/full-height/status-bar behavior with tests.

Validation

• cargo fmt --all -- --check
• cargo test --locked
• cargo build --release --locked
• cargo run --quiet -- --render --width 90 benchmark.md > /tmp/glass-render-full.ansi
• cargo run --quiet -- --render --width 90 --height 1 benchmark.md

[cowboycodr]

Closing in favor of the 0.1.8-scoped render/help branch.