feat(payments): add CEP-8 payment primitives

[harsh04044]

Part of #100

Adds the pure src/payments/ foundation for CEP-8 (capability pricing and payment over ContextVM). Types, tags, traits, and fakes only. No transport wiring, network, or negotiation; those come in follow-ups that build on this.

What's here

• Tag builders and parsers for cap, pmi, and payment_interaction (matches the ts-sdk vectors).
• Wire types for the transparent notifications (payment_required / accepted / rejected) and the explicit-gating error data (-32042 / -32043). Field order is pinned to the ts emitters and covered by exact-byte serde tests.
• PaymentProcessor, PaymentHandler, and ResolvePrice traits (async, object-safe), plus FakePaymentProcessor / FakePaymentHandler behind test-utils.
• PaymentError folded into the crate Error, with a dedicated CVM_PAYMENT = 8 FFI code (Rust enum, C header, and C test kept in lockstep).

Note

• ttl / retry_after are u64 seconds, so a malformed negative or fractional value fails closed at deserialize rather than being silently coerced. Reject tests cover it.
• No cap parser and no direct_payment / change producers, matching ts-sdk (both are optional or read-only on the client side).