Add gas/CPU instruction regression baselines per public entrypoint with CI diff gate

[Baskarayelu]

Description

QuickLendX exposes dozens of #[contractimpl] entrypoints in src/lib.rs (e.g. store_invoice, place_bid, accept_bid_and_fund, process_partial_payment, settle_invoice, distribute_revenue). Today there is no per-entrypoint Soroban CPU-instruction baseline checked into the repo, so silent gas regressions can slip through. This issue introduces a baseline table of (entrypoint, scenario) -> max_instructions/read_bytes/write_bytes captured via env.cost_estimate() / BudgetSnapshot, persisted in scripts/gas-baseline.toml, and enforced by a CI test that fails when any scenario exceeds the baseline by more than a configurable tolerance (default 3%).

Requirements and context

• Secure: ensure budget probes do not change protocol behaviour or skip require_auth.
• Tested: at least one baseline scenario per entrypoint listed in lib.rs #[contractimpl].
• Documented: docs/gas-baselines.md explaining how to refresh baselines and review diffs.
• Reference modules: src/lib.rs, src/escrow.rs, src/settlement.rs, src/bid.rs, src/profits.rs.

Suggested execution

git checkout -b feature/gas-regression-baselines

• Add src/bench.rs helper exposing measure(env, label, || { ... }) that records Budget deltas.
• Modify src/lib.rs only via #[cfg(test)] instrumentation (no behaviour change).
• Persist baselines in scripts/gas-baseline.toml; add scripts/update-gas-baseline.sh.
• Tests in tests/gas_regression.rs iterate scenarios and assert deltas within tolerance.
• Docs: docs/gas-baselines.md with run/update workflow.
• Use Rust doc /// NatSpec-style comments on the bench helpers.
• Validate that the bench harness cannot bypass require_auth or pause checks.

Test and commit

Run cargo test --test gas_regression -- --nocapture and capture output. Cover edge cases: paused-state rejection paths, error paths, and the largest realistic invoice. Include a security note that baselines are non-load-bearing for correctness.

Example commit message

test(gas): add per-entrypoint Soroban instruction-budget regression baselines

Guidelines

• Minimum 95% test coverage
• Clear documentation in docs/gas-baselines.md
• Timeframe: 96 hours