docs(nip-am): normative amendment — cache SHOULD/MUST + pricingIdentity + consumer cost guidance

[wpfleger96]

Amends docs/nips/NIP-AM.md with three normative publisher-behavior changes per the cleared Usage v2 plan (plan v3, D4 + D2').

Changes

1. Cache emission semantics (D4)

Replaces the unconditional MAY with qualified obligations:

• Publishers SHOULD emit cacheReadTokens / cacheWriteTokens when the provider exposes a cache component.
• Publishers MUST preserve an explicit zero when the provider reports zero.
• Publishers MUST omit the field (never null or fabricated zero) when that component is unavailable to the publisher — including when the provider supports it but the harness does not surface it.

An explicit carve-out in both the JSON comment block and the Numeric-validity prose exempts these fields from the payload-wide null guidance. Omission is the only valid representation for an unavailable cache component.

2. Optional pricingIdentity field (D2')

Adds an optional, non-nullable pricingIdentity object (authority, model, cacheClass), defined as billing authority — distinct from the transport Provider enum.

• authority is a registered billing-namespace identifier: exact lowercase hostname, no scheme, no path, no trailing slash. Registered values: api.anthropic.com, api.openai.com, openrouter.ai. The set extends only by NIP amendment. Pricing lookup is an exact string match on (authority, model).
• Present only when the publisher can prove applicability: direct official-endpoint connections prove via the actually-requested resolved model; other routes MUST receive response-supplied authoritative billing identity.
• MUST omit for custom/overridden base URLs, gateways (unless the gateway is the named billing authority), unresolved aliases, and turns where usage contributions carry more than one billing identity (including identity-bearing mixed with unresolved).
• cacheClass is omitted (not null) when not applicable.
• pricingIdentity is optional but not nullable — omission is the only absence representation.
• The existing model field retains its non-billing semantics (configured/session model) and is never overloaded.
• Consumers MUST treat omission as "price unknown" and MUST NOT infer a price from the session model field.

3. Consumer cost guidance (D4)

• Consumers MAY recompute cost estimates using the billing identity and a pricing manifest.
• Consumers MUST retain the provenance of any cost value (e.g. manifest-estimated, wire-reported).
• Consumers MUST NOT merge manifest-estimated and wire-reported costs into an unlabeled total.

Manifest-vs-wire display preference is application policy and deliberately excluded from this NIP.

Scope

Doc-only. Single file: docs/nips/NIP-AM.md.