[Enhancement] Implement SimulatedTx::assert_fee_below() and assert_instructions_below() helper methods

[ekelemepraise-code]

Summary

Currently users must manually extract and compare values from SimulatedTx:

let sim = env.simulate(|| client.call());
assert!(sim.fee() < 5000, "fee too high: {}", sim.fee());
assert!(sim.instructions() < 1_000_000, "too many instructions: {}", sim.instructions());

This is verbose and produces unhelpful panic messages. We should add fluent assertion methods directly on SimulatedTx<T>.

Proposed API

env.simulate(|| client.call())
   .assert_fee_below(5_000)           // fails with detailed message if fee >= 5000
   .assert_instructions_below(1_000_000)  // fails with detailed message if instructions >= threshold
   .commit();

Implementation Notes

• Methods should be chainable (return &Self or Self)
• Panic messages must include actual vs expected values
• Add corresponding assert_fee_above() / assert_instructions_above() for regression lower-bounds

Acceptance Criteria

• Both assertion methods implemented on SimulatedTx
• Chainable with .commit()
• Documented in API Reference section of README
• Example added to examples/