fix(highlight): make hl_eol work on background extmarks

Problem: hl_eol requires a multiline extmark to extend the background
past end-of-line. The extmark used end_col = line_len on the same
row, so it was single-line and hl_eol was effectively a no-op.

Solution: replace end_col with end_row targeting the next line so
the extmark is multiline and hl_eol takes effect. Split
number_hl_group into a separate extmark to prevent it bleeding to
adjacent lines.

lua/diffs/highlight.lua

@@ -458,12 +458,17 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
 
     if opts.highlights.background and is_diff_line then
       pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, {
-        end_col = line_len,
+        end_row = buf_line + 1,
         hl_group = line_hl,
         hl_eol = true,
-        number_hl_group = opts.highlights.gutter and number_hl or nil,
         priority = PRIORITY_LINE_BG,
       })
+      if opts.highlights.gutter then
+        pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, {
+          number_hl_group = number_hl,
+          priority = PRIORITY_LINE_BG,
+        })
+      end
     end
 
     if char_spans_by_line[i] then

spec/highlight_spec.lua

@@ -800,6 +800,72 @@ describe('highlight', function()
       delete_buffer(bufnr)
     end)
 
+    it('hl_eol background extmarks are multiline so hl_eol takes effect', 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.hl_group == 'DiffsAdd' or d.hl_group == 'DiffsDelete') then
+          assert.is_true(d.end_row > mark[2])
+        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 @@',
+        ' local a = 0',
+        '-local x = 1',
+        '+local y = 2',
+        ' local b = 3',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        start_line = 1,
+        lines = { ' local a = 0', '-local x = 1', '+local y = 2', ' local b = 3' },
+      }
+
+      highlight.highlight_hunk(
+        bufnr,
+        ns,
+        hunk,
+        default_opts({ highlights = { background = true, gutter = true } })
+      )
+
+      local extmarks = get_extmarks(bufnr)
+      for _, mark in ipairs(extmarks) do
+        local d = mark[4]
+        if d and d.number_hl_group then
+          local start_row = mark[2]
+          local end_row = d.end_row or start_row
+          assert.are.equal(start_row, end_row)
+        end
+      end
+      delete_buffer(bufnr)
+    end)
+
     it('line bg priority > DiffsClear priority', function()
       local bufnr = create_buffer({
         '@@ -1,2 +1,1 @@',