Use fail-closed WASM random backend

[prozacchiwawa]

Summary

• select getrandom_backend="unsupported" for the WASM build
• remove all direct getrandom 0.4/JavaScript-binding dependencies introduced by the earlier fix
• retain the existing MVP CPU target setting

Cause

The previous custom backend selection had no implementation, leaving an unresolved WASM env import that wasm-bindgen emitted as require("env").

Strategy

The unsupported backend is fail-closed: it introduces no JavaScript randomness interface and returns an explicit error if the WASM surface requests entropy. This avoids both an unresolved custom import and an insecure deterministic fallback.

The existing transitive dependency graph still contains getrandom 0.3 and 0.4 through clvmr's crypto dependencies, but neither receives the wasm_js feature and the package contains no JavaScript crypto bindings.

Validation

• built and packed with the exact workflow flags
• node wasm/tests/index.js passed
• clvm-js interface suite passed: 2 suites, 26 tests
• generated Node wrapper contains no getRandomValues, crypto, or require("env") binding