spec: define ACP ↔ ASP content, patch, identity, and fidelity mapping

[Airine]

Goal

Define the normative mapping required to ingest Agent Client Protocol (ACP) sessions into Agent Session Protocol (ASP) without silently losing semantics.

Scope

Specify:

• ACP content blocks ↔ ASP content/unknown blocks.
• Streaming session/update and patch application/order/idempotency.
• ACP SessionId ↔ ASP/consumer session identity and reconnect behavior.
• Tool calls/results, plans, usage, stop/end/disconnect semantics.
• ACP Other → ASP unknown opaque passthrough.
• Thinking/reasoning representation and mandatory fidelity-loss declaration when it cannot be expressed.
• Canonical-event representation expectations for consumers such as USL.
• Forward-compatibility and malformed/unknown update behavior.

Acceptance criteria

• Normative mapping table covers every current ACP update/content variant.
• Patch examples cover append, replace, duplicate, reorder, and reconnect.
• Identity rules distinguish ACP ids, ASP ids, and consumer-local ids.
• Fidelity axes and required loss declarations are unambiguous.
• Other and future variants have a lossless opaque preservation path.
• Examples include at least one real ACP harness transcript.
• USL capture implementation can reference a stable spec section/test vector.

Downstream

Blocks agent-session-protocol/universal-session-log#6.