Draft: add fog patch fragments and semantic seed

[mdheller]

Summary

This PR is the follow-on integration tranche after the merged FogVault / FogCompute schema scaffold PR.

It adds:

• openapi.fog.patch.yaml
• asyncapi.fog.patch.yaml
• docs/contract-additions/2026-04-fog-contract-index.md
• semantic/fog-vocabulary.jsonld

Why

The initial fog schema PR established object shapes. This PR makes those additions more discoverable and gives them dedicated additive API/event patch fragments plus a seed semantic vocabulary.

Included in this PR

API / event patch fragments

• dedicated additive OpenAPI fragment for fog surfaces
• dedicated additive AsyncAPI fragment for fog domain events

Discoverability

• fog contract additions index under docs/contract-additions/

Semantic seed

• minimal JSON-LD vocabulary seed for fog types and core predicates

Intentionally deferred

This PR does not yet patch the existing root catalog files directly:

• root README.md
• schemas/README.md
• examples/README.md
• semantic/README.md

Those files require direct in-place edits. This PR keeps the follow-on integration additive and non-destructive while preserving discoverability and machine-mergeable patch surfaces.

Follow-up

A subsequent cleanup PR should:

1. fold fog entries into the root README family table
2. add fog rows to schemas/README.md
3. add fog entries to examples/README.md
4. link the new semantic seed from the semantic index docs