Add FOC Platform receipt-aware upload runner path

[snissn]

Goal

Add or validate a production-oriented FOC Platform upload/coordinator runner path that can produce the receipt data required by the FOC Platform registry.

Parent tracker: FIL-Builders/foc-platform#3
Platform spec source: FIL-Builders/foc-platform spec.md section 6.7, section 10, and section 17.

Scope

Build on the current Filecoin upload scaffolding, remote adapter, and foc-process prototype to support a FOC Platform runner contract between generated app, upload adapter, coordinator, and Platform Contract:

• upload endpoint and status endpoint
• account id / relayer mapping
• object id or request id mapping
• declared size and content commitment
• requested copy count and CDN mode
• coordinator mode metadata
• finalization callback target
• receipt hash and copy receipt shape
• failure, retry, and polling semantics

The desired production direction is a hosted or SDK-backed coordinator that can return section 6.7-compatible UploadReceipt data. foc-process remains a prototype path unless validated for the deployment target.

Non-Goals

• Do not make Netlify/background upload production-supported without real deploy/runtime evidence.
• Do not implement the full registry contract in this issue.
• Do not expose secrets in generated artifacts.

Milestones

1. Define the FOC Platform upload runner result/receipt contract.
2. Add SDK-backed or coordinator-backed runner support, or document the exact gap blocking it.
3. Validate current Netlify/background upload behavior in a real or reproducible runtime, if it remains a recommended scaffold path.
4. Add integration tests for async upload status, success/failure result shape, and manifest wiring.
5. Document production vs prototype runner modes.

Required Evidence

• Integration test or reproducible smoke proof for the selected runner path.
• Documentation showing how generated apps map upload results into platform finalization receipts.
• Explicit runtime validation or limitation notes for Netlify/background upload scaffolding.

Completion Criteria

This issue is complete when Token Host Builder has a documented and tested FOC Platform upload/coordinator path that either produces section 6.7-compatible receipt data or clearly marks the remaining receipt gap.