feat: add custody-separated Peer Cash extension

[ADWilkinson]

Summary

Closes #1708.

• add @lucid-agents/peer-cash, a native Lucid extension that contributes runtime.peerCash
• cover capability discovery, 30-day corridor demand, market-rate estimates, unsigned cash-out preparation, receipt finalization, order reads, withdrawal, top-up, and restricted-platform access policy preparation
• add package documentation, the Lucid package reference, navigation, and a Changesets release entry for 0.1.0

Security boundary

This package does not accept private keys, sign transactions, or broadcast them. Mutating methods return JSON-safe unsigned Base transaction plans. The host wallet must inspect, sign, submit, and confirm each transaction in order. That keeps Lucid wallet connectors, account-abstraction policies, and external signers in control of custody.

Why this is Lucid-native

Lucid uses typed runtime slices. This implementation follows the current extension kernel, exposes one peerCash runtime directly, and leaves entrypoint selection and wallet policy to the host application.

Validation

• bun test — 1,498 passed, 58 environment-gated skips, 0 failed
• bun run build:packages
• bun run type-check
• bun run test:docs
• bun test packages\/peer-cash\/src\/__tests__\/peer-cash.test.ts — 11 passed
• package build and declaration generation
• package dry-run pack with all export targets present
• pre-commit lint, typecheck, and format gates

The tests exercise the public Lucid builder/runtime interface and mock only the external Peer SDK boundary.

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[ADWilkinson]

Quality review update:

• declared the extension as Node-only with Node.js 22 or newer, matching the Peer Cash SDK runtime instead of advertising browser portability
• widened the payee input to support structured identity-attestation data required by identity-gated payout handles
• added a structured-payee regression test and clarified the restricted-platform flow in the README

The 11 focused tests, typecheck, package build, portability check, and repository pre-commit gates pass. Both commits are GitHub-verified. The PR is mergeable and awaiting maintainer review.

[ADWilkinson]

Still current on this branch. Happy to rebase if the extension layout moved.