fix(highlight): support combined diff format for unmerged files

Problem: fugitive shows combined diffs (@@@ headers, 2-char prefixes)
for unmerged (UU) files. The parser and highlight pipeline assumed
unified diff format (@@, 1-char prefix), causing broken prefix
concealment, missing background colors on ` +`/`+ ` lines, and no
treesitter highlights due to garbage prefix chars in code arrays.

Solution: detect prefix width from the number of leading @ signs in
hunk headers. Propagate prefix_width through the parser (new field on
diffs.Hunk) and highlight pipeline (prefix stripping, col_offset,
concealment, line classification). Add U to filename pattern for
unmerged file detection. Skip intra-line diffing for combined diffs
since the 2-char prefix semantics don't produce meaningful change
groups.

spec/highlight_spec.lua

@@ -830,6 +830,114 @@ describe('highlight', function()
       delete_buffer(bufnr)
     end)
 
+    it('applies DiffsAdd background to combined diff lines with + in prefix', function()
+      local bufnr = create_buffer({
+        '@@@ -1,3 -1,5 +1,9 @@@',
+        '  local M = {}',
+        '++<<<<<<< HEAD',
+        ' +  return 1',
+        '++=======',
+        '+   return 2',
+        '++>>>>>>> feature',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        start_line = 1,
+        prefix_width = 2,
+        lines = {
+          '  local M = {}',
+          '++<<<<<<< HEAD',
+          ' +  return 1',
+          '++=======',
+          '+   return 2',
+          '++>>>>>>> feature',
+        },
+      }
+
+      highlight.highlight_hunk(
+        bufnr,
+        ns,
+        hunk,
+        default_opts({ highlights = { background = true } })
+      )
+
+      local extmarks = get_extmarks(bufnr)
+      local add_lines = {}
+      for _, mark in ipairs(extmarks) do
+        if mark[4] and mark[4].hl_group == 'DiffsAdd' then
+          add_lines[mark[2]] = true
+        end
+      end
+      assert.is_nil(add_lines[0])
+      assert.is_nil(add_lines[1])
+      assert.is_true(add_lines[2] ~= nil)
+      assert.is_true(add_lines[3] ~= nil)
+      assert.is_true(add_lines[4] ~= nil)
+      assert.is_true(add_lines[5] ~= nil)
+      assert.is_true(add_lines[6] ~= nil)
+      delete_buffer(bufnr)
+    end)
+
+    it('conceals 2-char prefix for combined diffs', function()
+      local bufnr = create_buffer({
+        '@@@ -1,2 -1,2 +1,3 @@@',
+        '  local M = {}',
+        '++<<<<<<< HEAD',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        start_line = 1,
+        prefix_width = 2,
+        lines = { '  local M = {}', '++<<<<<<< HEAD' },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts({ hide_prefix = true }))
+
+      local extmarks = get_extmarks(bufnr)
+      local overlay_count = 0
+      for _, mark in ipairs(extmarks) do
+        if mark[4] and mark[4].virt_text_pos == 'overlay' then
+          overlay_count = overlay_count + 1
+          assert.are.equal('  ', mark[4].virt_text[1][1])
+        end
+      end
+      assert.are.equal(2, overlay_count)
+      delete_buffer(bufnr)
+    end)
+
+    it('produces treesitter captures on combined diff content lines', function()
+      local bufnr = create_buffer({
+        '@@@ -1,2 -1,2 +1,3 @@@',
+        '  local M = {}',
+        ' +local x = 1',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        start_line = 1,
+        prefix_width = 2,
+        lines = { '  local M = {}', ' +local x = 1' },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      local has_ts = false
+      for _, mark in ipairs(extmarks) do
+        if mark[4] and mark[4].hl_group and mark[4].hl_group:match('^@.*%.lua$') then
+          has_ts = true
+          break
+        end
+      end
+      assert.is_true(has_ts)
+      delete_buffer(bufnr)
+    end)
+
     it('filters @spell and @nospell captures from injections', function()
       local bufnr = create_buffer({
         '@@ -1,1 +1,2 @@',