fix: clear stale gutter extmarks after fugitive section toggle

Problem: Repeatedly toggling `=` in fugitive left green gutter
extmarks on lines between sections. Two issues combined:
(1) carry_forward_highlighted returned an empty table (truthy in
Lua) when zero hunks matched, so pending_clear stayed false and
compressed extmarks from deleted diff lines were never cleared.
(2) The namespace clear in on_buf's pending_clear path was removed
in 2feb8a8, so even when pending_clear was true the extmarks
survived.

Solution: Return nil from carry_forward_highlighted when no hunks
were carried forward, so pending_clear is correctly set to true.
Restore nvim_buf_clear_namespace in on_buf's pending_clear block.
Add process_pending_clear test helper and spec coverage.

lua/diffs/init.lua

@@ -288,6 +288,9 @@ local function carry_forward_highlighted(old_entry, new_hunks)
     old_n,
     new_n
   )
+  if next(highlighted) == nil then
+    return nil
+  end
   return highlighted
 end
 
@@ -729,6 +732,7 @@ local function init()
       ensure_cache(bufnr)
       local entry = hunk_cache[bufnr]
       if entry and entry.pending_clear then
+        vim.api.nvim_buf_clear_namespace(bufnr, ns, 0, -1)
         entry.highlighted = {}
         entry.pending_clear = false
       end
@@ -927,12 +931,22 @@ function M.get_conflict_config()
   return config.conflict
 end
 
+local function process_pending_clear(bufnr)
+  local entry = hunk_cache[bufnr]
+  if entry and entry.pending_clear then
+    vim.api.nvim_buf_clear_namespace(bufnr, ns, 0, -1)
+    entry.highlighted = {}
+    entry.pending_clear = false
+  end
+end
+
 M._test = {
   find_visible_hunks = find_visible_hunks,
   hunk_cache = hunk_cache,
   ensure_cache = ensure_cache,
   invalidate_cache = invalidate_cache,
   hunks_eq = hunks_eq,
+  process_pending_clear = process_pending_clear,
 }
 
 return M

spec/decoration_provider_spec.lua

@@ -122,6 +122,31 @@ describe('decoration_provider', function()
       assert.is_true(entry.pending_clear)
       delete_buffer(bufnr)
     end)
+
+    it('clears namespace extmarks when on_buf processes pending_clear', function()
+      local bufnr = create_buffer({
+        'M test.lua',
+        '@@ -1,1 +1,2 @@',
+        ' local x = 1',
+        '+local y = 2',
+      })
+      diffs.attach(bufnr)
+      local ns_id = vim.api.nvim_create_namespace('diffs')
+      vim.api.nvim_buf_set_extmark(bufnr, ns_id, 0, 0, { line_hl_group = 'DiffAdd' })
+      assert.are.equal(1, #vim.api.nvim_buf_get_extmarks(bufnr, ns_id, 0, -1, {}))
+
+      diffs._test.invalidate_cache(bufnr)
+      diffs._test.ensure_cache(bufnr)
+      local entry = diffs._test.hunk_cache[bufnr]
+      assert.is_true(entry.pending_clear)
+
+      diffs._test.process_pending_clear(bufnr)
+
+      entry = diffs._test.hunk_cache[bufnr]
+      assert.is_false(entry.pending_clear)
+      assert.are.same({}, vim.api.nvim_buf_get_extmarks(bufnr, ns_id, 0, -1, {}))
+      delete_buffer(bufnr)
+    end)
   end)
 
   describe('BufWipeout cleanup', function()
@@ -262,7 +287,7 @@ describe('decoration_provider', function()
 
       local updated = diffs._test.hunk_cache[bufnr]
       assert.is_nil(updated.highlighted[1])
-      assert.is_false(updated.pending_clear)
+      assert.is_true(updated.pending_clear)
       delete_buffer(bufnr)
     end)
   end)