fix: restore core clarify boundary

[rlippmann]

What changed

• removed acquisition-derived pending replacement continuation from the core engine while preserving deterministic canonical replacement validation and clarify behavior
• updated controller, REPL, conformance fixtures, and structured regression expectations to match the restored core boundary
• kept checkpoint APIs but simplified the live contract so current core exports pending: null and rejects non-null pending checkpoint payloads
• aligned the directive grammar spec, API docs, architecture notes, README, and Demo 09 wording with the restored core clarify boundary

Why

• core should only handle canonical directives, deterministic validation against authoritative state, and deterministic update or clarify decisions
• the previous pending replacement flow asked core to reinterpret failed replacement directives into synthesized operations, which belongs in the future acquisition layer rather than the compiler core
• keeping the runtime, tests, fixtures, and docs aligned avoids leaking conversational intent-repair behavior back into the core contract

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)