fix(highlight): revert line backgrounds to hl_group+hl_eol (#124)

## Problem

The neogit commit (3d640c2) switched line background extmarks from
`hl_group`+`hl_eol` to `line_hl_group`. Due to [neovim#31151][1],
`line_hl_group` bg overrides `hl_group` bg regardless of extmark
priority. This made `DiffsAddText`/`DiffsDeleteText` intra-line
highlights invisible beneath line backgrounds — the extmarks were
placed correctly but Neovim rendered the line bg on top.

[1]: https://github.com/neovim/neovim/issues/31151

## Solution

Revert line backgrounds to `hl_group`+`hl_eol` where priority stacking
works correctly. Keep `number_hl_group` in a separate point extmark to
prevent gutter color bleeding to adjacent lines. The Neogit highlight
override (clearing their groups to `{}`) is independent and unaffected.

lua/diffs/highlight.lua

@@ -443,11 +443,18 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
     end
 
     if opts.highlights.background and is_diff_line then
-      local ext = { line_hl_group = line_hl, priority = p.line_bg }
+      pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, {
+        end_row = buf_line + 1,
+        hl_group = line_hl,
+        hl_eol = true,
+        priority = p.line_bg,
+      })
       if opts.highlights.gutter then
-        ext.number_hl_group = number_hl
+        pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, {
+          number_hl_group = number_hl,
+          priority = p.line_bg,
+        })
       end
-      pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, ext)
     end
 
     if is_marker and line_len > pw then

spec/highlight_spec.lua

@@ -287,7 +287,7 @@ describe('highlight', function()
       local extmarks = get_extmarks(bufnr)
       local has_diff_add = false
       for _, mark in ipairs(extmarks) do
-        if mark[4] and mark[4].line_hl_group == 'DiffsAdd' then
+        if mark[4] and mark[4].hl_group == 'DiffsAdd' then
           has_diff_add = true
           break
         end
@@ -320,7 +320,7 @@ describe('highlight', function()
       local extmarks = get_extmarks(bufnr)
       local has_diff_delete = false
       for _, mark in ipairs(extmarks) do
-        if mark[4] and mark[4].line_hl_group == 'DiffsDelete' then
+        if mark[4] and mark[4].hl_group == 'DiffsDelete' then
           has_diff_delete = true
           break
         end
@@ -386,7 +386,7 @@ describe('highlight', function()
       local extmarks = get_extmarks(bufnr)
       local has_diff_add = false
       for _, mark in ipairs(extmarks) do
-        if mark[4] and mark[4].line_hl_group == 'DiffsAdd' then
+        if mark[4] and mark[4].hl_group == 'DiffsAdd' then
           has_diff_add = true
           break
         end
@@ -500,7 +500,7 @@ describe('highlight', function()
       local extmarks = get_extmarks(bufnr)
       local has_diff_add = false
       for _, mark in ipairs(extmarks) do
-        if mark[4] and mark[4].line_hl_group == 'DiffsAdd' then
+        if mark[4] and mark[4].hl_group == 'DiffsAdd' then
           has_diff_add = true
           break
         end
@@ -560,7 +560,7 @@ describe('highlight', function()
       delete_buffer(bufnr)
     end)
 
-    it('uses line_hl_group for line backgrounds', function()
+    it('uses hl_group with hl_eol for line backgrounds', function()
       local bufnr = create_buffer({
         '@@ -1,2 +1,1 @@',
         '-local x = 1',
@@ -585,46 +585,14 @@ describe('highlight', function()
       local found = false
       for _, mark in ipairs(extmarks) do
         local d = mark[4]
-        if d and (d.line_hl_group == 'DiffsAdd' or d.line_hl_group == 'DiffsDelete') then
+        if d and (d.hl_group == 'DiffsAdd' or d.hl_group == 'DiffsDelete') and d.hl_eol then
           found = true
-          assert.is_nil(d.hl_eol)
         end
       end
       assert.is_true(found)
       delete_buffer(bufnr)
     end)
 
-    it('line_hl_group background extmarks are single-line', function()
-      local bufnr = create_buffer({
-        '@@ -1,2 +1,1 @@',
-        '-local x = 1',
-        '+local y = 2',
-      })
-
-      local hunk = {
-        filename = 'test.lua',
-        lang = 'lua',
-        start_line = 1,
-        lines = { '-local x = 1', '+local y = 2' },
-      }
-
-      highlight.highlight_hunk(
-        bufnr,
-        ns,
-        hunk,
-        default_opts({ highlights = { background = true } })
-      )
-
-      local extmarks = get_extmarks(bufnr)
-      for _, mark in ipairs(extmarks) do
-        local d = mark[4]
-        if d and (d.line_hl_group == 'DiffsAdd' or d.line_hl_group == 'DiffsDelete') then
-          assert.is_nil(d.end_row)
-        end
-      end
-      delete_buffer(bufnr)
-    end)
-
     it('number_hl_group does not bleed to adjacent lines', function()
       local bufnr = create_buffer({
         '@@ -1,3 +1,3 @@',
@@ -773,7 +741,7 @@ describe('highlight', function()
         if d then
           if d.hl_group == 'DiffsClear' then
             table.insert(priorities.clear, d.priority)
-          elseif d.line_hl_group == 'DiffsAdd' or d.line_hl_group == 'DiffsDelete' then
+          elseif (d.hl_group == 'DiffsAdd' or d.hl_group == 'DiffsDelete') and d.hl_eol then
             table.insert(priorities.line_bg, d.priority)
           elseif d.hl_group == 'DiffsAddText' or d.hl_group == 'DiffsDeleteText' then
             table.insert(priorities.char_bg, d.priority)
@@ -873,8 +841,9 @@ describe('highlight', function()
       local extmarks = get_extmarks(bufnr)
       local line_bgs = {}
       for _, mark in ipairs(extmarks) do
-        if mark[4] and mark[4].line_hl_group then
+        local d = mark[4]
-          line_bgs[mark[2]] = mark[4].line_hl_group
+        if d and (d.hl_group == 'DiffsAdd' or d.hl_group == 'DiffsDelete') and d.hl_eol then
+          line_bgs[mark[2]] = d.hl_group
         end
       end
       assert.is_nil(line_bgs[1])
@@ -1065,8 +1034,8 @@ describe('highlight', function()
       local marker_text = {}
       for _, mark in ipairs(extmarks) do
         local d = mark[4]
-        if d and d.line_hl_group then
+        if d and (d.hl_group == 'DiffsAdd' or d.hl_group == 'DiffsDelete') and d.hl_eol then
-          line_bgs[mark[2]] = d.line_hl_group
+          line_bgs[mark[2]] = d.hl_group
         end
         if d and d.number_hl_group then
           gutter_hls[mark[2]] = d.number_hl_group