feat(columns): per-character permission column highlights (#375) (#146)

Problem: the permissions column rendered as a monolithic unstyled
string, making it hard to scan `rwx` bits at a glance.

Solution: add per-character highlight groups for permission characters
following the `eza`/`lsd` convention. All groups link to standard
Neovim highlights so every colorscheme works out of the box.

lua/oil/adapters/files.lua

@@ -100,7 +100,8 @@ if not fs.is_windows then
       if not stat then
         return columns.EMPTY
       end
-      return permissions.mode_to_str(stat.mode)
+      local str = permissions.mode_to_str(stat.mode)
+      return { str, permissions.mode_to_highlights(str) }
     end,
 
     parse = function(line, conf)

lua/oil/adapters/files/permissions.lua

@@ -25,6 +25,31 @@ M.mode_to_str = function(mode)
     .. perm_to_str(bit.band(extra, 1) ~= 0 and 't', mode)
 end
 
+local char_hl = {
+  r = 'OilPermissionRead',
+  w = 'OilPermissionWrite',
+  x = 'OilPermissionExec',
+  s = 'OilPermissionSetuid',
+  S = 'OilPermissionSetuid',
+  t = 'OilPermissionSticky',
+  T = 'OilPermissionSticky',
+  ['-'] = 'OilPermissionNone',
+}
+
+---@param str string
+---@return table[]
+M.mode_to_highlights = function(str)
+  local highlights = {}
+  for i = 1, #str do
+    local c = str:sub(i, i)
+    local hl = char_hl[c]
+    if hl then
+      table.insert(highlights, { hl, i - 1, i })
+    end
+  end
+  return highlights
+end
+
 ---@param mode integer
 ---@return string
 M.mode_to_octal_str = function(mode)

lua/oil/adapters/ssh.lua

@@ -118,7 +118,11 @@ local ssh_columns = {}
 ssh_columns.permissions = {
   render = function(entry, conf)
     local meta = entry[FIELD_META]
-    return meta and permissions.mode_to_str(meta.mode)
+    if not meta then
+      return
+    end
+    local str = permissions.mode_to_str(meta.mode)
+    return { str, permissions.mode_to_highlights(str) }
   end,
 
   parse = function(line, conf)

lua/oil/init.lua

@@ -1066,6 +1066,36 @@ M._get_highlights = function()
       link = 'OilHidden',
       desc = 'Hidden executable files in an oil buffer',
     },
+    {
+      name = 'OilPermissionRead',
+      link = 'DiagnosticWarn',
+      desc = 'Read permission character in the permissions column',
+    },
+    {
+      name = 'OilPermissionWrite',
+      link = 'DiagnosticError',
+      desc = 'Write permission character in the permissions column',
+    },
+    {
+      name = 'OilPermissionExec',
+      link = 'DiagnosticOk',
+      desc = 'Execute permission character in the permissions column',
+    },
+    {
+      name = 'OilPermissionSetuid',
+      link = 'DiagnosticError',
+      desc = 'Setuid/setgid permission character in the permissions column',
+    },
+    {
+      name = 'OilPermissionSticky',
+      link = 'DiagnosticInfo',
+      desc = 'Sticky bit permission character in the permissions column',
+    },
+    {
+      name = 'OilPermissionNone',
+      link = 'NonText',
+      desc = 'No permission character in the permissions column',
+    },
     {
       name = 'OilCreate',
       link = 'DiagnosticInfo',