docs(keywords): specify keyword library v1 contract

[hubab1]

Summary

• specify an add-only local Keyword Library with explicit app, storefront, and platform scope
• cap library writes and preview fan-out; forbid implicit all-app/all-storefront propagation
• define exact Unicode normalization, opaque V1 key snapshots, canonical preview digests, idempotency, cancellation, and explicit resume semantics
• define immutable provenance, append-only outcomes, cross-process writer coordination, bounded retries, retention, and privacy rules
• bound native and read-only MCP paging, including the final encoded JSON-RPC response size

Source and attribution

This is an independent redesign informed by these commits from the open WIP PR #8:

• akshaynexus@ee8c34e
• akshaynexus@9da616b

The links record design provenance only; no code or commit authorship was copied.

Validation

• markdownlint: 0 issues
• all JSON examples parse
• references and fenced blocks validate
• all linked standards/source URLs return successfully
• two independent P0/P1/P2 review rounds completed; final review clear

Review gate

Draft/manual approval required before implementation because this contract introduces persistence, cross-process mutation coordination, quotas, and future remote-surface policy.