Remove SES-lockdown workarounds in evm-wallet-experiment now that vats can request crypto/Math

[sirtimid]

Context

Once #935 lands and vats can request crypto, SubtleCrypto, and Math in their globals array, several workarounds in packages/evm-wallet-experiment become obsolete. They were added because SES Compartments previously had no access to crypto.getRandomValues or the stock Math.random.

What to simplify

1. Drop the throwaway-keyring entropy option (high value)

makeKeyring({ type: 'throwaway', entropy?: Hex }) in src/lib/keyring.ts exists only so callers can supply entropy externally when the vat has no crypto global. With crypto endowed in the keyring vat, the workaround is dead weight.

Files touched:

• src/lib/keyring.ts — drop the entropy branch, the globalThis.crypto?.getRandomValues guard, and the "throwaway keyring requires…" error path; tighten the KeyringInitOptions type
• src/lib/keyring.test.ts — remove the entropy-validation and crypto-required test cases
• src/vats/coordinator-vat.ts — remove entropy from the initializeKeyring RPC signature / KeyringInitOptions plumbing
• src/vats/keyring-vat.ts — verify it still works without the plumbed-through entropy
• src/cluster-config.ts — add 'crypto' and 'SubtleCrypto' to the keyring vat's globals (this is the change that unblocks the rest)
• scripts/setup-home.sh, scripts/setup-away.sh, scripts/home-interactive.mjs — drop the openssl rand-based entropy generation and stop passing entropy into initializeKeyring
• test/e2e/docker/helpers/wallet-setup.ts — mirror the cluster-config changes
• README.md (root of the package) — remove the "Throwaway keyring needs secure entropy" caveat
• docs/setup-guide.md — remove the "Under SES lockdown…" entropy section

2. Simplify makeSaltGenerator (medium value)

src/lib/delegation.ts::makeSaltGenerator has a crypto-available branch and a counter-fallback branch. With crypto endowed, the fallback becomes unreachable.

• Collapse to a single crypto-only implementation
• Drop the optional entropy parameter
• Remove the counter-fallback tests (keep a test that two generators produce distinct salts)
• Update the JSDoc that describes the SES fallback

The coordinator-vat wiring at coordinator-vat.ts:377-383 can also drop its explanatory comment.

3. Reconsider raw-fetch clients vs viem (speculative, lower priority)

src/lib/bundler-client.ts and src/lib/provider.ts reimplement JSON-RPC over raw fetch to avoid viem's createClient/createPublicClient, which internally use Math.random(). With Math endowed in the provider vat, viem's clients should work — but the raw-fetch wrappers are small and focused, and migration is a refactor rather than a simplification. Recommend evaluating whether the ergonomic wins from viem's higher-level clients (retries, batching, typed RPC methods) are worth the churn; if not, just drop the "Math.random() is blocked under SES lockdown" comments that will no longer be true.

Depends on

• Integrate Snaps attenuated endowment factories into vat globals #935 (integrates the Snaps attenuated endowment factories that provide crypto, SubtleCrypto, and Math)