feat: enable vim syntax fallback by default (#152)

## Problem

Languages without a treesitter parser (COBOL, Fortran, etc.) got no
syntax highlighting because \`highlights.vim.enabled\` defaulted to
\`false\`.

## Solution

Flip the default to \`true\`. The vim syntax path is already deferred
via \`vim.schedule\` so it never blocks the first paint. \`max_lines =
200\` stays unchanged — appropriate given the ~30x slower per-hunk cost
vs treesitter.

README.md

@@ -15,11 +15,11 @@ with language-aware syntax highlighting.
   [vscode-diff](https://github.com/esmuellert/codediff.nvim) FFI backend for
   word-level accuracy)
 - `:Gdiff` unified diff against any revision
-- Background-only diff colors for `&diff` buffers
 - Inline merge conflict detection, highlighting, and resolution
-- Email-quoted diff highlighting (`> diff ...` prefixes, arbitrary nesting
-  depth)
-- Vim syntax fallback, configurable blend/priorities
+- Email quoting/patch syntax support (`> diff ...`)
+- Vim syntax fallback
+- Configurable highlighiting blend & priorities
+- Context-inclusive, high-accuracy highlights
 
 ## Requirements