feat: add server-owned summary-only compaction v3

[acastellana]

What changed

• Adds a strict, stateless POST /v1/compact contract v3 that accepts only an optional raw previous summary and newly aged complete interaction groups.
• Keeps authority messages, the retained tail, framing, layout, and splice decisions in the client.
• Uses an operator-owned compact profile with one routed attempt, bounded input/output, validated provider completion and usage metadata, and path-owned operation:compact authorization.
• Records billing and safe operation metadata without persisting messages, summaries, previous summaries, or decision traces.

Why

The earlier contract made the router responsible for too much client context layout and pushed defensive state into SubZeroClaw. The v3 boundary makes the router a narrowly scoped summarization transform, so the runtime can stay synchronous and lean while retaining local authority over exactly what enters its model context.

Validation

• 548 passed, 2 skipped in the Python suite.
• 5 passed in antseed/db.test.js.
• Real HTTP E2E from the SubZeroClaw binary through the FastAPI compact endpoint, including an applied byte-reducing seal.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/semantic-context-compaction

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