perf(bot): evaluate Rust/WASM migration for Sloth VDF challenge path

## Rationale
"The one exception would be if you add cryptographic proof-of-work verification in the bot challenge module. Even then, crypto.subtle (built into all edge runtimes) handles that natively and is already implemented in C++/Rust under the hood."

This is very hard issue, i failed inthe past to do it.

## Goal
Evaluate migrating Sloth VDF compute/verify hot path from TypeScript to Rust (compiled to WASM) for lower latency and better throughput in bot challenge mode.

## Scope
- Benchmark current TypeScript Sloth VDF implementation under realistic edge constraints.
- Prototype Rust implementation targeting WASM.
- Compare TS vs Rust/WASM for compute latency, verify latency, bundle impact, and cold start.
- Keep a stable TypeScript fallback path for environments where WASM is restricted.

## Acceptance Criteria
- Benchmark report with methodology and results.
- Decision document: keep TS vs hybrid vs full Rust/WASM path.
- If adopted: feature-flagged integration and tests with coverage >= 80 percent metrics.