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/lib.lua

@@ -8,6 +8,9 @@ local cached_handle = nil
 ---@type boolean
 local download_in_progress = false
 
+---@type fun(handle: table?)[]
+local pending_callbacks = {}
+
 ---@return string
 local function get_os()
   local os_name = jit.os:lower()
@@ -164,9 +167,10 @@ function M.ensure(callback)
     return
   end
 
+  table.insert(pending_callbacks, callback)
+
   if download_in_progress then
-    dbg('download already in progress')
-    callback(nil)
+    dbg('download already in progress, queued callback')
     return
   end
 
@@ -192,21 +196,25 @@ function M.ensure(callback)
   vim.system(cmd, {}, function(result)
     download_in_progress = false
     vim.schedule(function()
+      local handle = nil
       if result.code ~= 0 then
         vim.notify('[diffs] failed to download libvscode_diff', vim.log.levels.WARN)
         dbg('curl failed: %s', result.stderr or '')
-        callback(nil)
-        return
+      else
+        local f = io.open(version_path(), 'w')
+        if f then
+          f:write(EXPECTED_VERSION)
+          f:close()
+        end
+        vim.notify('[diffs] libvscode_diff downloaded', vim.log.levels.INFO)
+        handle = M.load()
       end
 
-      local f = io.open(version_path(), 'w')
-      if f then
-        f:write(EXPECTED_VERSION)
-        f:close()
+      local cbs = pending_callbacks
+      pending_callbacks = {}
+      for _, cb in ipairs(cbs) do
+        cb(handle)
       end
-
-      vim.notify('[diffs] libvscode_diff downloaded', vim.log.levels.INFO)
-      callback(M.load())
     end)
   end)
 end