Small additions, fixes, and dropping "experimental" status. 🎉

Changelog

New features

Add -d/--diff-file to start diff panel from CLI.
In-place saving for large files: edits can be written back without spilling
to a temp copy, and your undo history survives the save (capped by a max
volume so it stays sane).
Capability-based enforcement for documents, so operations that a document
type can't support are blocked cleanly. This will be used in a future version
for disk and process type documents.

Editing past the 4 GiB mark of very large files now works (fixed a bad
32-bit skip calculation).
Replacing exactly at end-of-file correctly extends the document instead of
leaving a gap.
Reads near EOF no longer count uninitialized bytes as real data.
Undo/redo fixes: branching off a new edit after undo now properly discards
the stale redo tail, the "edited" indicator stays accurate across save points,
and several coalescing edge cases (empty-document removes, double-split
buffers) were corrected so undo can't corrupt unrelated history.

Upgraded terminal module to handle multibyte POSIX and UnicodeChar on Windows.
- It's now possible to bind fancier keybindings, like to € (U+20AC).
Add enter, spacebar, escape, and end special keys to be bound with.
Added more members in Key enum.
- Fixes cast(Key)91 in --help-keys. Instead, you'll see LeftBracket.

ddhx.1: Added CLI, commands, and color sections; updated color elements/list; general proofreading.
README: Removed the experimental notice, fixed test-color Makefile typo.