Add opt-in Nishvault pre-send receipt before XMTP 402 payment broadcast

[cakirasim]

This adds an opt-in guard immediately before the existing USDC transfer broadcast in src/chain/xmtp402.ts.

When NISHVAULT_PRE_SEND_GUARD=1 is set, the payment flow becomes:

402 payment details -> build USDC transfer calldata -> PRE_SEND_PROOF_RECEIPT -> sendTransaction

Default behavior is unchanged because the guard is disabled unless the env var is set.

Configuration:

NISHVAULT_PRE_SEND_GUARD=1
NISHVAULT_SELLER_URL=https://api.nishvault.com
X402_BUYER_PRIVATE_KEY=0x...

Why this fits this repo:

• This repo already targets Base mainnet.
• xmtp402.ts already has the exact broadcast slot.
• The patch does not add a runtime dependency; it calls npx --package nishvault-preflight-buy nishvault-guard-tx only when explicitly enabled.
• The guard fails closed on non-Base-mainnet chain IDs.

Validation:

npm ci --ignore-scripts
npm run typecheck

Note: the full repo typecheck currently fails on existing unrelated missing modules/config typing errors. A targeted tsc invocation for src/chain/xmtp402.ts reaches only existing src/config.ts strictness errors and does not report errors from the Nishvault patch.