fix: include segments_count bytes in worst_case_body_size (M8)

[QuantumExplorer]

Summary

Fixes audit finding M8: worst_case_body_size underestimates prefix hash input by 8 bytes (on 64-bit platforms).

Problem

build_prefix_body constructs a body consisting of:

1. All segment bytes concatenated
2. segments_count.to_ne_bytes() — a usize (8 bytes on 64-bit)
3. One u8 length byte per segment

But worst_case_body_size only accounted for #1 and #3:

path.len() + path.iter().map(|a| a.max_length() as usize).sum::<usize>()
// = segments_count + sum(segment_bytes)
// Missing: size_of::<usize>() for segments_count.to_ne_bytes()

This underestimate means a path with body size in [57, 64] would be estimated as needing 1 Blake3 block (≤64 bytes) when it actually requires 2 blocks (>64 bytes), causing fee cost estimation to undercharge.

Fix

Add std::mem::size_of::<usize>() to the computation:

path.iter().map(|a| a.max_length() as usize).sum::<usize>()
    + std::mem::size_of::<usize>()
    + path.len()

Test plan

• All storage tests pass (cargo test -p grovedb-storage)
• All 1274 grovedb lib tests pass (cargo test -p grovedb --lib)

🤖 Generated with Claude Code

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⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: ad5829ec-7cf8-4b14-9fc5-04d72f46fee8

📥 Commits

Reviewing files that changed from the base of the PR and between a42c936 and 784dbc6.

📒 Files selected for processing (1)

• storage/src/rocksdb_storage/storage.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/M8-worst-case-body-size

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.66%. Comparing base (1b119d3) to head (784dbc6).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #565      +/-   ##
===========================================
- Coverage    90.66%   90.66%   -0.01%     
===========================================
  Files          182      182              
  Lines        49474    49491      +17     
===========================================
+ Hits         44856    44869      +13     
- Misses        4618     4622       +4     

Components	Coverage Δ
grovedb-core	88.86% <ø> (-0.02%)	⬇️
merk	91.88% <ø> (ø)
storage	85.50% <100.00%> (+0.02%)	⬆️
commitment-tree	96.41% <ø> (ø)
mmr	96.72% <ø> (ø)
bulk-append-tree	91.50% <ø> (ø)
element	97.55% <ø> (ø)

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[QuantumExplorer]

Reviewed