⚡ Bolt: Optimize DocxXMLEditor change ID generation

[timteh]

💡 What: Caches the _next_change_id in DocxXMLEditor to prevent rescanning the entire DOM for every new tracked change insertion.

🎯 Why: The previous implementation scanned all w:ins and w:del elements to find the maximum w:id every time a new change was added. This resulted in O(N^2) complexity for batch insertions (e.g., applying redlines).

📊 Impact:

• Reduces complexity from O(N^2) to O(N) for batch operations.
• 11x faster for 1000 insertions (Benchmark: ~2.9s -> ~0.26s).
• 6.5x faster for 500 insertions (Benchmark: ~0.72s -> ~0.11s).

🔬 Measurement:

• Verified with a reproduction script that inserts 1000 tracked changes.
• Verified that generated IDs remain unique and sequential.
• Verified that manual ID overrides are respected and update the cache correctly.

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PR created automatically by Jules for task 6118911914792885974 started by @timteh

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