Add receipt export consumer for gitea-sovereign audit log

[mdheller]

Context

gitea-sovereign is the source-control substrate and emits source-control receipts. It must not own the canonical evidence ledger.

The intended export path is:

Audit log -> export cursor -> model-governance-ledger

model-governance-ledger remains the canonical evidence and receipt consumer authority.

Required consumer contract

Add a consumer contract for gitea-sovereign receipt export batches that can accept, verify, and acknowledge source-control receipt exports.

Expected producer-side artifacts currently exist in SocioProphet/gitea-sovereign:

• schemas/receipt-export.schema.json
• core/receipt-export.js
• docs/adr/0003-audit-chain-and-export.md
• docs/adr/0006-runtime-binding-gates.md

Consumer requirements

• Accept export batch metadata from gitea-sovereign.
• Verify batch identity, sequence range, event count, and batch hash.
• Preserve acknowledgement hash semantics.
• Fail closed for malformed batches, replayed/duplicate batches, sequence gaps, hash mismatch, unsupported version, or producer identity mismatch.
• Store canonical evidence/receipt records under ledger-owned semantics.
• Do not grant source-control authority back to gitea-sovereign.

Non-goals

• No source-control write authority in the ledger.
• No policy/grant authority in the ledger.
• No Git transport implementation in the ledger.

Dependency

This blocks live ledger binding from gitea-sovereign; until this consumer contract exists, gitea-sovereign must remain local/export-scaffold only.