Undo/redo with typing coalescing for a text buffer (word boundaries, 2-second gap, backspace vs forward-delete groups, history cap)

# Undo/redo with typing coalescing for a text buffer `editor.py` is the text buffer behind our notes app. Users expect one undo to remove a whole typed word, not one letter. Rewrite `Editor` (Python 3 stdlib only) with this exact behaviour. API: `Editor(text="")`, attribute `.text`, `insert(pos, s, t)`, `delete(pos, n, t)`, `undo() -> bool`, `redo() -> bool`. `t` is the edit time in seconds (float; compare with plain subtraction `t - last <= 2.0`; the hidden tests never use times where floating-point rounding could change that comparison). ## Edits 1. `insert`: `pos` must be an int with `0 <= pos <= len(text)`, else `ValueError`. Inserting `""` changes nothing and is not recorded (but positions and time are still validated). 2. `delete(pos, n, t)` removes `text[pos:pos+n]`; `pos` and `n` must be ints with `n >= 1`, `pos >= 0`, `pos + n <= len(text)`, else `ValueError`. 3. `t` must never be smaller than the `t` of the previous recorded edit, else `ValueError` (nothing changes). Validation errors leave everything unchanged. ## Undo groups 4. Every recorded edit either **joins** the most recent undo group or starts a new one. A new edit can only join if: the most recent group is still **open** (no `undo()`/`redo()` call happened since that group was last changed), the group was created by a **single-character** edit (a multi-character insert or delete is a group of its own that never accepts more edits), the new edit is a single character, `t - (time of the group's last edit) <= 2.0`, and the group currently holds **fewer than 20 characters**. 5. **Typing:** a 1-char insert joins an insert group if `pos` equals group start + group text length (typing right after it) and the group's last character is not a space or `\n` (a space/newline is still added to the group, but ends it for further typing). 6. **Deleting:** a 1-char delete joins a delete group either as **backspace** (`pos + 1 ==` group start; the group start moves to `pos`) or as **forward delete** (`pos ==` group start). A group's direction is fixed by its second delete; after that only the same direction joins. 7. Insert never joins a delete group and vice versa. 8. `undo()` reverts the most recent group as a whole and moves it to the redo stack; `redo()` re-applies the most recently undone group. Both return `False` (and change nothing else, except closing the open group) when their stack is empty, else `True`. Any recorded edit (including one that joins a group) clears the redo stack. 9. The undo stack keeps at most **50** groups; when a 51st is pushed (by a new group or by `redo`), the oldest is dropped. ## Scoring Hidden check groups (partial credit = fraction passed; all must pass): `validation` (1-3), `typing_groups` (4-5), `word_boundaries` (5), `time_gap` (4: exactly 2.0 s joins, 2.0001 s does not), `delete_directions` (6-7), `undo_redo_semantics` (8), `group_limits` (4: 20-char cap; 9: 50 groups), `randomized_agreement` (500 random sequences of 60 operations: `.text` and every return value must match the rules after each step).