fix(delta): add content-based validation to block-level operations (prevents silent overwrites)

[handnewb]

Summary

Fixes #3273: In delta-mode mental-model refresh, ReplaceBlockOp and RemoveBlockOp targeted blocks by LLM-computed integer index with range-only validation. A wrong-but-in-range index silently overwrote or removed an unrelated block, and the refresh committed it as success.

Root Cause

The apply_operations function validated only that the index was within [0, len(blocks)) — no content check. If the LLM's computed index drifted (e.g., because the document changed since the LLM last saw it, or the LLM miscounted), the operation corrupted the document silently.

Solution

New expected_block field on ReplaceBlockOp and RemoveBlockOp:

• Optional — None by default (fully backward compatible)
• When set, the operation is only applied if section.blocks[index] serialises to the same JSON as expected_block
• On mismatch, the operation is skipped with a clear reason ("index drift detected") — the document stays intact
• New _blocks_equal() helper compares via model_dump_json()

Impact

• LLM index drift no longer causes silent data corruption
• Backward compatible: existing prompts that don't emit expected_block see no behavior change
• The audit trail (skipped list) now surfaces drift events for debugging
• Progressive enhancement: updated LLM prompts can emit expected_block for full protection