Add gas cost benchmarks for all contract entrypoints to track Soroban fee regression

[ALLEN-AYODEJI]

Description

Soroban has per-operation gas costs that affect user fees. Currently there are no benchmarks measuring gas consumption for each entrypoint. A gas cost regression would let developers see if a PR increases user costs. This is critical for maintaining affordable contract interaction.

Requirements and context

• Security: Standard security review required for state-mutating changes
• Testing: Full test suite must pass; new tests for changed behavior
• Documentation: Update doc comments and README if user-facing

Suggested execution

Branch: fix/feat/test-gas-benchmarks

Implement changes

• Create a benches/contract_bench.rs file using Soroban's test infrastructure
• Add benchmark functions for: init (1 and 10 milestones), lock_funds, submit_work, approve_and_release, raise_dispute, resolve_dispute
• Print gas costs in a machine-parseable format (JSON) for CI comparison
• Add a CI step that compares gas costs against the baseline and warns on >10% increase

Test and commit

Run benchmarks and record baseline gas costs. Verify the benchmark output is parseable. Make a small change and verify the diff is detected.

Example commit message

feat(test): add Soroban gas cost benchmarks for all 8 contract entrypoints

Guidelines

• Open a PR against master
• Keep PR focused on this single concern
• Run cargo test or npm test before review
• Ensure no lint or typecheck warnings