[Ingestion]: ZIP archive expansion — ADR needed for fan-out contract

[inherent-prime]

Problem

Agents often hold a bundle (repo export, docs folder) as one ZIP; today every member must be uploaded individually.

Open design questions — decide via a short ADR before implementation

• Fan-out contract: one upload → many documents breaks DocumentUploadMessage (one document per event, contracts v1.0.0). Child documents with parent_document_id? New event?
• Dedup: member-level content_hash vs archive-level.
• Limits: member count, nesting depth (zip-in-zip), zip-bomb guards (compression-ratio cap).
• Partial failure semantics: 3 of 10 members fail — parent status? Must respect the compensation rules (mark-failed via compensation.py, no state/response divergence).

Proposal

• Write the ADR answering the questions above; implement only after [Ingestion]: File-type support registry — single contract for validation, sniffing, extraction, and docs #117 lands.
• Members route through the [Ingestion]: File-type support registry — single contract for validation, sniffing, extraction, and docs #117 registry like normal uploads; unsupported members are skipped and recorded in parent metadata (never silently dropped).
• Surface: REST-only.

Acceptance criteria

• ADR merged under docs/adr/.
• Implementation issue(s) filed from the ADR decisions.

Depends on #117.