feat(buffer): track line changes via on_bytes to keep _meta aligned

Problem: `_meta` is a positional array keyed by line number. Line
insertions and deletions during editing desync it from actual buffer
content, breaking `get_fold()`, cursor-based task lookups, and extmark
re-application.

Solution: attach an `on_bytes` callback that adjusts `_meta` on line
insertions/deletions and tracks dirty rows. Remove the manual
`_meta` insert from `open_line()` since `on_bytes` now handles it.
Reset dirty rows on each full render.

lua/pending/buffer.lua

@@ -26,6 +26,10 @@ local _initial_fold_loaded = false
 local _filter_predicates = {}
 ---@type table<integer, true>
 local _hidden_ids = {}
+---@type table<integer, true>
+local _dirty_rows = {}
+---@type boolean
+local _on_bytes_active = false
 
 ---@return pending.LineMeta[]
 function M.meta()
@@ -95,6 +99,48 @@ function M.clear_marks(b)
   vim.api.nvim_buf_clear_namespace(bufnr, ns_inline, 0, -1)
 end
 
+---@return table<integer, true>
+function M.dirty_rows()
+  return _dirty_rows
+end
+
+---@return nil
+function M.clear_dirty_rows()
+  _dirty_rows = {}
+end
+
+---@param bufnr integer
+---@return nil
+function M.attach_bytes(bufnr)
+  if _on_bytes_active then
+    return
+  end
+  _on_bytes_active = true
+  vim.api.nvim_buf_attach(bufnr, false, {
+    on_bytes = function(_, buf, _, start_row, _, _, old_end_row, _, _, new_end_row, _, _)
+      if buf ~= task_bufnr then
+        _on_bytes_active = false
+        return true
+      end
+      local delta = new_end_row - old_end_row
+      if delta > 0 then
+        for _ = 1, delta do
+          table.insert(_meta, start_row + 2, { type = 'task' })
+        end
+      elseif delta < 0 then
+        for _ = 1, -delta do
+          if _meta[start_row + 2] then
+            table.remove(_meta, start_row + 2)
+          end
+        end
+      end
+      for r = start_row + 1, start_row + 1 + math.max(0, new_end_row) do
+        _dirty_rows[r] = true
+      end
+    end,
+  })
+end
+
 ---@return nil
 function M.persist_folds()
   log.debug(
@@ -208,7 +254,6 @@ function M.open_line(above)
   local insert_row = above and (row - 1) or row
   vim.bo[bufnr].modifiable = true
   vim.api.nvim_buf_set_lines(bufnr, insert_row, insert_row, false, { '- [ ] ' })
-  table.insert(_meta, insert_row + 1, { type = 'task' })
   vim.api.nvim_win_set_cursor(0, { insert_row + 1, 6 })
   vim.cmd('startinsert!')
 end
@@ -452,6 +497,7 @@ function M.render(bufnr)
   end
 
   _meta = line_meta
+  _dirty_rows = {}
 
   snapshot_folds(bufnr)
   vim.bo[bufnr].modifiable = true
@@ -511,6 +557,7 @@ function M.open()
   if not (task_bufnr and vim.api.nvim_buf_is_valid(task_bufnr)) then
     task_bufnr = vim.api.nvim_create_buf(true, false)
     set_buf_options(task_bufnr)
+    M.attach_bytes(task_bufnr)
   end
 
   vim.cmd('botright new')