Add Phoenix transfer WASM benchmark

[HDauven]

Summary

• add a plonk-transfer-wasm crate that builds a real Phoenix transfer prover artifact for browser WASM
• expose the standard plonkweb ABI (plonkweb_init, plonkweb_prove, plonkweb_verify) so the artifact can be loaded by plonkjs
• wire a threaded transfer WASM build target with 4 GiB shared-memory limit and optional wasm-opt pass
• document the TxCircuitFourTwo fixture, key source, trusted unchecked prover-key loading, and backend override path

Notes

• Depends on the plonk-private browser-wasm-transfer-bench branch for the prover hot-path and trusted unchecked key loading API.
• The browser harness and ppk/preprocessing experiments are intentionally not included.

Testing

• cargo fmt --all -- --check
• make build-transfer-wasm-rayon