fix: clear stale gutter extmarks after fugitive section toggle (#139)

## Problem

Repeatedly toggling `=` in fugitive left green gutter
(`number_hl_group`)
extmarks on lines between sections. When fugitive collapses a diff
section,
Neovim compresses extmarks from deleted lines onto the next surviving
line
(the `M ...` file entry). Two issues prevented cleanup:

1. `carry_forward_highlighted` returned `{}` (truthy in Lua) when zero
hunks
matched, so `pending_clear` stayed `false` and the compressed extmarks
   were never cleared.
2. The `nvim_buf_clear_namespace` call 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 (`next(highlighted) == nil`), 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

@@ -247,7 +247,7 @@ end
 
 ---@param old_entry diffs.HunkCacheEntry
 ---@param new_hunks diffs.Hunk[]
----@return table<integer, true>
+---@return table<integer, true>?
 local function carry_forward_highlighted(old_entry, new_hunks)
   local old_hunks = old_entry.hunks
   local old_hl = old_entry.highlighted
@@ -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