feat: add IGRA-Kaspa stress test runner

[reshmem]

Summary

• add the IGRA load generator for deterministic stress campaigns with wallet/contract mapping, warmup, metrics, manifests, calibration/preflight modes, and pass/fail evaluation
• add Kaspa fan-out support with standardness validation and adaptive batch splitting
• add stress-test prep docs, generated wallet/allocation fixtures, and smoke/stress helper scripts

Validation

• cargo fmt --package igra-loadgen --check
• cargo test -p igra-loadgen
• git diff --check

Notes

• This PR targets the IGRA support branch, not upstream master.
• Live devnet/testnet stress execution remains the next acceptance gate.