feat: add negotiated fulfillment location context

[igrigorik]

This PR supersedes #642, which identified the need to preserve the Business location behind store-scoped Catalog results through Cart and Checkout. Instead of adding #642's raw location field to base Context, this PR makes support explicit through Fulfillment.

• adds optional context.location to Fulfillment's Catalog, Cart, and Checkout projections;
• gives Cart location-aware Context without full Fulfillment structures;
• introduces one opaque, stable, Business-scoped Location Identifier;
• defines field roles, cross-Business isolation, unknown-ID handling, and precedence.

Adding a Business-defined location ID to base Context would define its syntax without advertising which Businesses recognize it, which operations consume it, or what behavior it affects. Fulfillment owns the relevant surfaces: Catalog methods report resolved locations, Catalog filters name destinations, and Checkout exposes destinations and selections. Contributing context.location through Fulfillment lets parties negotiate each parent capability with extends.

Contract

The same Location Identifier can appear in several fields without giving them the same meaning:

Field	Meaning
context.location	Provisional, non-binding anchor for location-scoped pricing, availability, estimates, or initial options.
Catalog filters.fulfills_to	Explicit destination filter: can this product be fulfilled to here?
variants[].fulfillment.methods[].location	Location for which a place-based Catalog method was resolved.
Checkout selection or authoritative address	Explicit destination for that scope; supersedes context.location.

Journey example...

Catalog reports method.location = loc_123
        ↓
Cart carries context.location = loc_123
        ↓
Cart-to-Checkout preserves context.location
        ↓
Checkout offers loc_123 as an initial choice
        ↓
Platform selects selected_destination_id = loc_123
  → loc_123 becomes explicit for that fulfillment scope

The new common/types/location_id.json reuses the existing string wire type across Fulfillment context, Catalog method locations, destination filters, and retail locations. Within one Business, the same value identifies the same location; its containing field determines its role. selected_destination_id stays generic because it can select other destination types.

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Checklist

• Capability: New schemas (Discovery, Cart, etc.) or extensions.
• Documentation: Updates to README, or documentations regarding schema or capabilities.
• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.