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
 

doc/diffs.nvim.txt

@@ -89,7 +89,7 @@ Configuration is done via `vim.g.diffs`. Set this before the plugin loads:
           max_lines = 500,
         },
         vim = {
-          enabled = false,
+          enabled = true,
           max_lines = 200,
         },
         intra = {
@@ -204,7 +204,7 @@ Configuration is done via `vim.g.diffs`. Set this before the plugin loads:
                              See |diffs.TreesitterConfig| for fields.
 
         {vim}                (table, default: see below)
-                             Vim syntax highlighting options (experimental).
+                             Vim syntax fallback highlighting options.
                              See |diffs.VimConfig| for fields.
 
         {intra}              (table, default: see below)
@@ -269,13 +269,15 @@ Configuration is done via `vim.g.diffs`. Set this before the plugin loads:
 
                                                           *diffs.VimConfig*
     Vim config fields: ~
-        {enabled}            (boolean, default: false)
+        {enabled}            (boolean, default: true)
                              Use vim syntax highlighting as fallback when no
                              treesitter parser is available for a language.
                              Creates a scratch buffer, sets the filetype, and
                              queries |synID()| per character to extract
-                             highlight groups. Slower than treesitter but
-                             covers languages without a TS parser installed.
+                             highlight groups. Deferred via |vim.schedule()|
+                             so it never blocks the first paint. Slower than
+                             treesitter but covers languages without a TS
+                             parser installed (e.g., COBOL, Fortran).
 
         {max_lines}          (integer, default: 200)
                              Skip vim syntax highlighting for hunks larger than

lua/diffs/init.lua

@@ -124,7 +124,7 @@ local default_config = {
       max_lines = 500,
     },
     vim = {
-      enabled = false,
+      enabled = true,
       max_lines = 200,
     },
     intra = {