Implement a compute budget estimation entry point for all major operations

[Kingsman-99]

Overview

Add a simulation-only entry point that returns the estimated compute units (CPU instructions + memory bytes) for each major contract operation given its inputs, without mutating state. This allows SDK/frontend callers to pre-estimate fees before submitting a real transaction.

Requirements

• estimate_compute(env, operation: Symbol, params: Map<Symbol, Val>) -> ComputeEstimate read-only entry point
• ComputeEstimate { cpu_insns: u64, mem_bytes: u64, fee_stroops: i128 } return type
• Support estimating: create_invoice, pay, release, refund, open_dispute, approve_release
• Use Soroban's env.budget() API to capture actual instruction counts during simulation
• Write tests asserting estimates are within 10% of actual measured costs
• Document each operation's typical budget range in docs/COMPUTE_BUDGETS.md

Acceptance Criteria

• estimate_compute returns valid estimates for all 6 operations
• Estimates within 10% of actual costs verified in tests
• docs/COMPUTE_BUDGETS.md updated with measured ranges
• cargo test --workspace passes with zero failures
• cargo clippy --all-targets -- -D warnings produces zero warnings

Definition of Done

All CI checks must pass before the PR is reviewed.

[nurudeenheroic]

I'd appreciate the opportunity to contribute to this task. I'll approach it carefully and make sure the implementation is polished and dependable.