feat: add memory leak monitoring for load tests

[shaaibu7]

Add memory leak monitoring for long-running load tests

Description
This PR adds memory leak monitoring to the PropChain load-test suite so long-running endurance runs can detect RSS growth and teardown
drift, not just throughput regressions.

What changed

• Added a lightweight process RSS sampler to tests/load_tests.rs
• Added memory growth reporting and bounded leak assertions for endurance/scalability sessions
• Added new long-running test coverage:
  • endurance_test_short
  • endurance_test_sustained_load
  • scalability_test_memory_usage
• Updated the load-test runners to invoke the correct library test targets
• Updated load-testing docs to reflect the new endurance/memory monitoring workflow and thresholds

Behavior

• Endurance tests now record peak RSS during execution
• Tests fail if memory growth or final RSS drift exceeds the configured budget
• Summaries now include peak memory usage in MB

Docs updated

• docs/LOAD_TESTING_GUIDE.md
• docs/LOAD_TEST_QUICK_START.md
• docs/LOAD_TEST_MONITORING.md

Verification

• cargo fmt --all --check
• Attempted cargo test -p propchain-tests endurance_test_short --release -- --nocapture
• Full test compile is currently blocked by an unrelated existing syntax error in contracts/property-token/src/types.rs

closes #159

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