feat(auth): Privy embedded wallet → Cosmos signer adapter

[Sentinel-Bluebuilder]

Summary

Privy provides embedded EVM/Solana wallets but has no native Cosmos signer. This PR adds an adapter that bridges a Privy-held key to a cosmjs OfflineDirectSigner, so consumers can use Privy for auth/onboarding while still calling every Sentinel SDK helper that takes a wallet (connect, broadcast, broadcastWithFeeGrant, operator helpers, etc.).

Two strategies

Picked by the mode field on createPrivyCosmosSigner.

Mode A — mnemonic (seed-import)

Consumer triggers Privy's exportWallet() once. The adapter re-derives a Cosmos secp256k1 key on the standard m/44'/118'/0'/0/0 path and wraps it in DirectSecp256k1HdWallet. Trust model is identical to a normal mnemonic wallet.

const signer = await PrivyCosmosSigner.fromMnemonic({ mnemonic: privyExportedSeed });

Mode B — rawSign (custody-preserving)

Seed never leaves Privy. Consumer supplies the compressed pubkey + a signRawSecp256k1(digest32) callback. The adapter computes sha256(makeSignBytes(signDoc)) locally and ships only the 32-byte digest. Privy's raw secp256k1 sign endpoint returns (r||s); the adapter normalizes to low-S (cosmos-sdk rejects high-S since v0.42).

const signer = await PrivyCosmosSigner.fromRawSign({
  pubkey: privyDerivedCompressedPubkey,
  signRawSecp256k1: async (digest32) => privy.signRawHash({ hash: digest32, curve: 'secp256k1' }),
});

Both modes derive the same sent1... address from the same seed.

What's exported

• PrivyCosmosSigner (static facade with fromMnemonic, fromRawSign, create, derivePubkeyFromMnemonic)
• PrivyRawSignDirectSigner (the OfflineDirectSigner class)
• privyCosmosSignerFromMnemonic, privyCosmosSignerFromRawSign, createPrivyCosmosSigner
• deriveCosmosPubkeyFromMnemonic — precompute the sent1... address during Privy onboarding before the signer is wired up

All wired through index.js.

Tests

test/privy-cosmos-signer.test.mjs — 20 assertions covering:

• Mode A address parity with createWallet()
• deriveCosmosPubkeyFromMnemonic matches Mode A
• Mode B address parity with Mode A using the same seed
• signDirect produces a signature that verifies against the pubkey on sha256(makeSignBytes(signDoc))
• High-S signatures returned by the callback are normalized to low-S, and the normalized form still verifies
• signerAddress mismatch is rejected with a helpful message
• Unified factory routes correctly and rejects unknown modes
• Static facade delegates to the underlying functions

No network, no Privy SDK in the test — the raw-sign callback is simulated locally with cosmjs Secp256k1.createSignature. The contract this PR establishes is what a real Privy raw-sign endpoint must satisfy.

20 passed, 0 failed
401 exports OK

Docs

docs/PRIVY-INTEGRATION.md — usage for both modes, address parity guarantees, requirements on the Privy callback (raw 32-byte digest, no eth_sign-style prefixing, must use Cosmos coinType 118), failure-mode table.

Test plan

• node test/privy-cosmos-signer.test.mjs — 20/20 pass
• node -e "import('./index.js').then(...)" — 401 exports
• node test/smoke.js — no new failures (only the pre-existing SDK_VERSION is 1.0.0 failure unrelated to this change)
• Live-chain: optional follow-up, since the unit test verifies the signature against the pubkey end-to-end