types: Evidence has no structured receipt field — receipts ride as JSON strings in detail

[Sharathvc23]

Problem

Evidence (nest_core/types.py:299) has no structured payload field:

class Evidence(BaseModel):
    reporter: AgentId
    subject: AgentId
    kind: str
    detail: str = ""
    timestamp: float | None = None

Trust plugins that work from signed receipts have to smuggle them through the string field. agent_receipts (merged in #26) says it outright in its module docstring:

NEST's Evidence has no receipt field, so a cross-signed receipt rides as JSON in evidence.detail.

and its report() does json.loads(evidence.detail), then falls back to the plain-string heuristic on decode failure (agent_receipts.py:616-644). The parc plugin (#63) inherits the same convention. Every producer has to json.dumps a dict into a string field and every consumer has to try-parse it back, with a silent semantic fork between "string that happens to be JSON" and "string that is prose".

The same shape keeps being reinvented per layer because there is no shared primitive:

• payments has its own Receipt model (types.py:126) — unrelated to trust evidence;
• datafacts' cid_facts defines a signed FreshnessProof carrying a Signature;
• trust receipts are untyped dicts with issuer_did / action / signature / evidence.witness_signatures keys, spec'd only by the code that verifies them.

Proposal

Add an optional structured payload to Evidence, non-breaking (defaults keep every existing caller valid):

Option A — minimal (my preference):

class Evidence(BaseModel):
    ...
    receipt: dict[str, Any] | None = None

Consumers prefer receipt when present and keep the detail JSON fallback for one release. No schema commitment beyond "a receipt-shaped dict"; the verifying plugin still owns validation, exactly as today.

Option B — typed: promote the receipt dict convention from agent_receipts into a SignedReceipt model in nest_core.types (receipt_id, issuer_did, action, signature, witness co-signatures). Stronger contract, but it freezes a wire format that currently belongs to one plugin family, and pydantic validation on hostile receipts would need care to keep "invalid receipt" a scoring decision rather than a parse exception.

Option C — generic: attachments: list[dict[str, Any]] for arbitrary structured evidence. Most flexible, least discoverable.

Option A is the smallest change that removes the string-smuggling without deciding a universal receipt schema prematurely; B can follow later if more plugins converge on one shape.

Scope if accepted

• Evidence.receipt field + docstring/example update in nest_core/types.py
• agent_receipts.report() and parc.report() prefer the structured field, keep the detail fallback
• receipt_reputation / parc_migration scenarios switch their auditors to the structured field
• tests for both paths (structured, fallback, and structured-but-invalid)

Happy to send the PR if the direction (and which option) is agreed.