Add Markdown preview mode

[Gordon Lam (yeelam-gordon)]

Summary

• add a read-only rendered Markdown preview for Markdown documents;
• toggle preview with View > Markdown Preview or Alt+P;
• keep preview state and caching independent per document;
• reuse Edit's TUI, framebuffer, Unicode measurement, scrolling, colors, and localization systems;
• add no crate dependencies.

Architecture

The implementation separates:

• markdown_preview/parser.rs: block and inline parsing;
• markdown_preview/layout.rs: Unicode-aware wrapping and rendered lines;
• markdown_preview.rs: per-document cache and lifecycle;
• draw_markdown_preview.rs: TUI drawing only.

The parser is bounded to preserve Edit's responsiveness:

• 1 MiB maximum preview source;
• 32 KiB maximum line length;
• O(1) cache fast path for unchanged buffers;
• raw-byte validation after generation reuse;
• bounded forward scanning for malformed links and backtick runs.

Tests

The business-logic tests cover:

• supported block and inline formatting;
• malformed and incomplete input;
• invalid UTF-8;
• Unicode wrapping and style preservation;
• cache reuse and content invalidation;
• undo/edit generation reuse;
• per-document state and scroll resets;
• large-file and long-line limits;
• adversarial bracket and backtick inputs with bounded-work assertions.

Clean-checkout validation:

cargo fmt --all -- --check
cargo clippy --workspace --all-features --all-targets -- --no-deps --deny warnings
cargo test --workspace --all-targets
cargo build --release -p edit

Size

Upstream release binary: 427,520 bytes
Preview release binary:  446,976 bytes
Difference:               +19,456 bytes (+4.55%)

Demo

Limitations

• lightweight Markdown subset rather than full CommonMark;
• links are rendered as text and are not clickable;
• one line over 32 KiB disables preview for that document;
• localization currently falls back to English for the three new strings.

Closes 923

[Gordon Lam (yeelam-gordon)]

Closing because I mistakenly opened this draft against the upstream repository. Development and review will continue in the fork PR: yeelam-gordon#1