Add benchmark for metadata write cost as key count grows #615

Description

Measure set_metadata cost at 1, 5, and MAX_METADATA_KEYS key counts to detect superlinear growth in the metadata storage layout. Publishes to CI for regression tracking.

Requirements and context

• Must be secure, tested, and documented
• Should be efficient and easy to review
• Relevant code: contracts/subscription_vault/src/metadata.rs
• Bench lives in contracts/subscription_vault/benches/metadata_write.rs

Suggested execution

• Fork the repo and create a branch
• git checkout -b bench/metadata-write
• Implement changes
  • Parametrise bench over key counts
  • Record read+write costs separately
  • Enforce ceiling in CI
• Validate security and correctness assumptions

Test and commit

• Run tests
  • cargo test --all
• Cover edge cases
  • Key at MAX length, value at MAX length, and idempotent overwrite
• Include test output and security notes

Example commit message

bench: metadata write cost by key count

Guidelines

• Minimum 95 percent test coverage
• Clear documentation
• Timeframe: 96 hours