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