fix: restore selection when undoing the last history item

[fendermoon]

Summary

Fixes #1100.

Stack::pop() decreases the logical item count but keeps the row containing its
remote delta alive for cursor restoration. When the popped item was the last
one, transform_based_on_this_delta() returned early because self.is_empty()
checks that logical count. The subsequent cursor transform therefore used the
remote delta without rebasing it through the undo.

Check self.stack.is_empty() instead. This keeps the empty-storage guard while
allowing the retained row's delta to be transformed. It does not introduce a new
history or change the merge algorithm.

The regression starts with Hello world!, commits it, and then creates the
UndoManager. It deletes the selected ello, imports a peer's Hi and ii
insertions, and undoes the deletion. The text must be Hi Helloii world! and
the absolute selection positions delivered in on_pop metadata
(CursorWithPos.pos.pos) must be [4,8], not [4,10]. Resolving the retained
cursors through doc.get_cursor_pos(...).current.pos already returns [4,8]
before the fix; the regression distinguishes these two results.

The same test first runs a control with one unrelated map edit in the history.
The control passes without the fix; the last-item arm fails. Neither arm loads
a snapshot or calls clear() before editing. Both also check fresh snapshot
and peer readback of the document value and version vector.

Validation

Base: d9ddfba195f9363653ac1d83e744641dcb337f4e.
Prepared commit: 7db750e38d61fb83200d9f9665931e77bce5fad7.

• Before the fix: the new test fails with older_item=false at the on_pop
  metadata assertion, actual [4,10], expected [4,8]. The older-item control,
  text/readback checks and cursor-resolution assertion pass first.
• After the fix: cargo test --locked -p loro -p loro-internal --lib --tests -- --test-threads=1
  passes 1,050 tests; 12 existing tests are ignored. Native macOS, Rust 1.94.0,
  dev debug information disabled and incremental compilation disabled.
• The focused test also passes after committing the patch.
• WASM/browser tests, release-mode performance and fuzzing were not run.

Includes the repository-required patch changeset. No public API or storage
format changes are proposed.