fix(highlight): make intra-line bg visible under line backgrounds

Problem: `line_hl_group` bg unconditionally overrides `hl_group` bg
regardless of priority (neovim/neovim#31151). `DiffsAddText`/
`DiffsDeleteText` at p201 were invisible under `DiffsAdd`/`DiffsDelete`
`line_hl_group` at p200 because they operate on separate stacking layers.

Solution: replace `line_hl_group` with `hl_group` + `hl_eol` +
`end_row` for line backgrounds, putting them on the same layer as
intra-line highlights so priority governs stacking. Split
`number_hl_group` into a separate extmark to prevent gutter bleed via
`end_row`. Use raw `DiffAdd.bg`/`DiffDelete.bg` for `DiffsAddText`/
`DiffsDeleteText` instead of alpha-blending toward Normal.bg, which
produced indistinguishable colors on dark themes.

lua/diffs/highlight.lua

@@ -637,10 +637,17 @@ 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, {
-          line_hl_group = line_hl,
-          number_hl_group = opts.highlights.gutter and number_hl or nil,
+          end_row = buf_line + 1,
+          hl_group = line_hl,
+          hl_eol = true,
           priority = p.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 = p.line_bg,
+          })
+        end
       end
 
       if is_marker and line_len > pw then

lua/diffs/init.lua

@@ -347,6 +347,9 @@ end
 ---@param path string
 ---@return string[]?
 local function read_file_lines(path)
+  if vim.fn.isdirectory(path) == 1 then
+    return nil
+  end
   local f = io.open(path, 'r')
   if not f then
     return nil
@@ -540,11 +543,10 @@ local function compute_highlight_groups(is_default)
   local normal_fg = normal.fg or (dark and 0xcccccc or 0x333333)
 
   local alpha = config.highlights.blend_alpha or 0.6
-  local text_alpha = math.min(alpha + 0.3, 1.0)
   local blended_add = blend_color(add_bg, bg, alpha)
   local blended_del = blend_color(del_bg, bg, alpha)
-  local blended_add_text = blend_color(add_bg, bg, text_alpha)
-  local blended_del_text = blend_color(del_bg, bg, text_alpha)
+  local blended_add_text = add_bg
+  local blended_del_text = del_bg
 
   local clear_hl = { default = dflt, fg = normal_fg }
   if not transparent then

spec/context_spec.lua

@@ -165,6 +165,39 @@ describe('context', function()
       assert.is_nil(hunks[1].context_after)
     end)
 
+    it('skips when path is a directory', function()
+      vim.fn.mkdir(vim.fs.joinpath(tmpdir, 'subdir'), 'p')
+
+      local hunks = {
+        make_hunk('subdir', {
+          file_new_start = 1,
+          file_new_count = 1,
+          lines = { '+x' },
+        }),
+      }
+      compute_hunk_context(hunks, 25)
+
+      assert.is_nil(hunks[1].context_before)
+      assert.is_nil(hunks[1].context_after)
+    end)
+
+    it('skips when path is a symlink to a directory', function()
+      vim.fn.mkdir(vim.fs.joinpath(tmpdir, 'real_dir'), 'p')
+      vim.uv.fs_symlink(vim.fs.joinpath(tmpdir, 'real_dir'), vim.fs.joinpath(tmpdir, 'link_dir'))
+
+      local hunks = {
+        make_hunk('link_dir', {
+          file_new_start = 1,
+          file_new_count = 1,
+          lines = { '+x' },
+        }),
+      }
+      compute_hunk_context(hunks, 25)
+
+      assert.is_nil(hunks[1].context_before)
+      assert.is_nil(hunks[1].context_after)
+    end)
+
     it('skips when file does not exist on disk', function()
       local hunks = {
         make_hunk('nonexistent.lua', {