feat: in-process markdown preview when cmux isn't running

[nero408]

Make cmux optional. If a user doesn't have cmux installed (or passes --no-cmux), mdmux now renders the markdown panel itself inside its own ratatui surface — a horizontal split with the file tree on the left and the rendered markdown on the right. For cmux users, nothing changes.

Why

The previous launch story was "you need cmux running, or the TUI lets you browse but Enter shows an error overlay." Setting up cmux is a non-trivial install — gating an open-source release on it shrinks the audience to "people who already use cmux", which is the wrong direction for a Day-1 release. Glow is a fine standalone renderer, but spawning it would either need its own pane (defeating the purpose) or popping out of the TUI entirely. The right answer is to render inside our pane.

What changed

New modules

• src/preview.rs — Preview struct holds a snapshot of an open file; load() reads from disk with a 1 MiB / 5000-line cap; render() produces a Text<'static> via tui-markdown for ratatui. Size cap protects us from accidentally pointing the binary at a giant log file.
• src/watcher.rs — FileWatcher wraps notify for live-reload. We watch the parent directory (not the file itself) so editors that save via rename-then-move don't break the watch. Events drain through a channel that the event loop checks between key presses; no polling drain when no preview is open.

App state

• New RenderMode enum (Cmux | InProcess) on App. Decided at startup based on cmux ping success + CLI flags. Defaults to Cmux.
• Replaced the demo-only DemoPreview / demo_preview field with a general-purpose preview: Option<Preview> plus preview_scroll: u16. --demo keeps its fake-surface-id behavior via the existing demo_mode flag, but the in-process pane is now first-class.
• New App::reload_preview, preview_scroll_{up,down,to_top,to_bottom}.

CLI

• New --no-cmux flag: force in-process even when cmux is available.
• Auto-fallback: when cmux ping fails we silently switch to InProcess instead of popping the "cmux not running" modal. Status line communicates which mode we ended up in.

Rendering

• Dropped the bespoke 80-line markdown renderer in src/ui.rs. The new Preview::render() uses tui-markdown (which uses pulldown-cmark under the hood), giving us headings, lists, blockquotes, fenced code blocks with syntect-powered syntax highlighting, and inline emphasis for free.
• New draw_preview widget; the demo-pane code path now serves real users, not just gif rendering.

Keys

• J / K scroll the in-process preview down / up.
• Ctrl-D / Ctrl-U scroll a half-page.
• Help screen + README updated.

Event loop

• Watcher integrates via a 200 ms poll timeout only when InProcess mode has a file open. Cmux mode and idle in-process mode block forever as before. Worst-case CPU on an idle in-process session is 5 wakeups/s to check the watcher — cheap, and cmux users pay nothing.

Dependencies

• tui-markdown = "=0.3.6" (pinned: 0.3.7+ bumps to the ratatui 0.30 workspace, which is a different ratatui-core crate and incompatible with our 0.29).
• notify = "8" (stable 8.2.0).

Binary size: ~1.5 MiB → ~3.8 MiB. The extra mass is syntect's bundled syntax/theme set, which is what gives us code-block highlighting.

Tests

• 12 new unit tests (5 preview, 3 watcher, 4 app)
• cargo test --lib: 48 passed, 0 failed
• cargo clippy --all-targets --all-features -- -D warnings: clean
• cargo fmt --all -- --check: clean

Known transitive cargo-audit warnings — accepted

All four are unmaintained / unsound markers on transitive deps with no exploit path for us:

• paste 1.x (ratatui dep, informational)
• lru 0.12 IterMut (ratatui internal; we don't call IterMut)
• bincode 1.x (syntect dep, 1.x stale but functional)
• yaml-rust 0.4 (syntect dep, unmaintained)

All clear after upgrading to ratatui 0.30 + tui-markdown 0.3.7, which is a separate, more invasive change best handled after the launch settles.