fix(mft): apply snapshot/restore pattern to directory index merge

[githubrobbi]

Summary

Fixes remaining LIVE parser parity mismatches by applying the same snapshot/restore pattern used for $DATA to directory index sizes.

Root cause identified: Directory index merging used unconditional += at direct_index_extension.rs:742-743, causing data loss when IOCP delivered extension records before base records.

Impact: Reduced mismatches from 16,517 → 422 with the previous $DATA fix (commit e90aade). This fix is expected to resolve the remaining 422 mismatches by applying the same proven pattern to directory index sizes.

Changes

• ✅ Apply snapshot/restore to dir_index merge (direct_index_extension.rs:737-766)
• ✅ Add chaos test harness for reproducing LIVE out-of-order scenarios
• ✅ Add regression tests for extension-before-base merging
• ✅ Add CHAOS_TEST_HARNESS.md documentation

Validation

• ✅ All 116 tests pass (OFFLINE correctness preserved)
• ✅ Code formatted and linted (ultra-strict Clippy rules)
• ⏳ Ready for Windows LIVE validation

Technical Details

The Bug

When extension arrives before base:

1. Extension adds dir_index to first_stream.size (0 + ext = ext) ✅
2. Base overwrites with = SizeInfo {...}, losing extension data ❌

The Fix

Check if first_stream.size is empty (0, 0):

• If empty → write extension's dir_index values
• Otherwise → accumulate using saturating_add

This is the same pattern that successfully fixed the $DATA bug in commit e90aade.

Test Plan

• All existing tests pass
• Code formatted (just fmt)
• Code linted (just lint-prod)
• Windows LIVE validation against D: drive (next step)

Related Issues

• Follows up on PR fix(mft): preserve extension first_stream.size and dir_index in LIVE parser #6 (fix-f-drive-parity)
• Addresses small directory size deltas (+51, +11 bytes)
• Resolves extension-before-base merge hazards

🤖 Generated with coordination from Claude Code Coordinator