RFC 0019: host.blobStorage + host.cache capabilities

Summary

Adds two sibling capabilities: host.blobStorage (binary artifact store with presigned URLs) and host.cache (TTL cache for HTTP / AI response memoization). Required by core.openwop.storage's blob-_and cache-_ nodes.

Motivation

Workflow editors universally surface object storage (S3 / GCS / Azure Blob) and HTTP response caching as discoverable primitives. The two are small enough and similar enough in shape to share an RFC. host.cache is what lets idempotency-key based replay deduplicate identical AI calls across runs without engaging the heavier Layer-2 invocation log.

Proposal

§A Capability schemas

{
  "blobStorage": {
    "type": "object",
    "properties": {
      "supported": { "type": "boolean" },
      "buckets": { "type": "array" },
      "presignSupported": { "type": "boolean" },
      "maxObjectBytes": { "type": "integer" }
    },
    "additionalProperties": false
  },
  "cache": {
    "type": "object",
    "properties": {
      "supported": { "type": "boolean" },
      "maxValueBytes": { "type": "integer" },
      "maxTtlSeconds": { "type": "integer" }
    },
    "additionalProperties": false
  }
}

§B Host-contract MUSTs

1. Blob: cross-tenant isolation per bucket. Presigned URLs MUST expire at the advertised TTL. 2. Cache: TTL drift ≤ 1s on expiry visibility; entries scoped per tenant.

§C Conformance

• blob-roundtrip.test.ts.
• blob-presign-expiry.test.ts.
• blob-cross-tenant-isolation.test.ts.
• cache-ttl-expiry.test.ts.
• cache-cross-tenant-isolation.test.ts.

Compatibility

Additive. Two optional blocks. Packs refuse registration without advertisement.

Implementation notes (non-normative)

• Schema diffs in §A land in schemas/capabilities.schema.json on Active promotion, not at Draft.
• Cross-tenant invariants for blob-cross-tenant-isolation + cache-cross-tenant-isolation land in SECURITY/invariants.yaml alongside the matching scenarios.
• Reference impl candidate: per-tenant Map for cache; per-tenant Map<bucket, Map<key,buffer>> for blob — both under examples/hosts/in-memory/.

Acceptance criteria

• [x] Two capability blocks.
• [x] Prose section per block. (Landed at commit c5831fe.)
• [x] 5 conformance scenarios.
• [x] In-memory reference (per-tenant Map for cache; per-tenant Map<bucket, Map<key,buffer>> for blob).
• [x] CHANGELOG entry.

References

• AWS S3 / GCS / Azure Blob (prior art for blob).
• HTTP Cache-Control / ETag semantics (prior art for cache).
• core.openwop.storage pack.