perf: cache repo root and harden async paths

Problem: get_repo_root() shells out on every call, causing 4-6
redundant subprocesses per gdiff_file() invocation.
highlight_vim_syntax() leaks a scratch buffer if nvim_buf_call errors.
lib.ensure() silently drops callbacks during download, permanently
missing intra-line highlights. The debounce timer callback can operate
on an invalid buffer.

Solution: Cache get_repo_root() results by parent directory. Wrap
nvim_buf_call and nvim_buf_delete in pcall so the scratch buffer is
always cleaned up. Queue pending callbacks in lib.ensure() so all
callers receive the handle once the download completes. Guard the
debounce timer callback with nvim_buf_is_valid.

lua/diffs/init.lua

@@ -200,7 +200,9 @@ local function create_debounced_highlight(bufnr)
           timer = nil
           t:close()
         end
-        highlight_buffer(bufnr)
+        if vim.api.nvim_buf_is_valid(bufnr) then
+          highlight_buffer(bufnr)
+        end
       end)
     )
   end