Refresh numbered list labels immediately after Enter

[JSv4]

Enter and Backspace now refresh existing numbered-list labels immediately, including Roman formats, custom starts, and separate restarts. The update preserves marker wrappers, spacing, and paragraph DOM.

Docxodus 12.6.2 also retains stale native list counters after splits/merges, so structural edits recompute labels in a short-lived read-only native snapshot. Ordinary text edits and bullet-only lists skip this work; live history and settings remain unchanged.

Closes #45.

Validation:

• Confirmed the regression observed 1, 2, 2, 3 synchronously after Enter; parameterized browser tests now verify correct labels before a full render, Backspace, restarts, Roman formatting, and retained marker DOM.
• Confirmed and fixed unnecessary snapshots on ordinary paste; regression cases cover plain paste beside a numbered list and splitting a bullet item.
• npm run check passes (lint, 82 unit tests, types, library/demo builds, API checks).
• Full browser run: 57 passed, 2 optional stress tests skipped; 5 targeted cases passed again after the review remediation.
• Adversarial review with gpt-5.6-sol identified the unnecessary snapshot work; remediation received a clean re-review.