fix(undoRedo): clear redo stack when using limit option

[sgup]

Problem

When the limit option is set on undoRedo(), making a change after undoing does not clear the redo stack. This causes history corruption where undone states persist and can be incorrectly restored.

Steps to reproduce:

1. Create undoRedo with a limit: undoRedo(obs$, { limit: 10 })
2. Make changes: A → B → C
3. Undo (now at B, can redo to C)
4. Make a new change: B → D
5. Bug: Redo stack still contains C, history is corrupted

Root Cause

The original code had mutually exclusive branches:

if (options?.limit) {
    // Only keeps last N items - does NOT clear redo stack!
    history = history.slice(Math.max(0, history.length - options.limit));
} else {
    // Correctly clears redo stack
    history = history.slice(0, historyPointer + 1);
}

Solution

Always clear redo history first, then apply the limit separately:

// Always clear any redo history first (items after current position)
history = history.slice(0, historyPointer + 1);

// Then apply the limit if specified
if (options?.limit) {
    history = history.slice(Math.max(0, history.length - options.limit));
}

Testing

Added regression test 'Undo/Redo with limit clears redo stack on new change' that verifies the fix.

All existing tests pass.