Companion to PARQUET_PUSHDOWN_DESIGN.md. Maps the design's six phases
to thirteen shippable milestones, with the architectural decisions
needed to start coding (residency, wire protocol, trust mode,
predicate engine scope, side-index path, library choices) called out
as defaults that the open-questions section asks to confirm.
No code changes — this commit only adds the plan document. M0 does
not start until the open questions are resolved.
