REST scan planning: local-vs-remote parity test (Phase 6 follow-up)

[zeroshade]

Follow-up to #1494 (part of #1178, Phase 6).

#1494 landed a scenario-driven, client-conformance fake REST scan-planning server at catalog/rest/internal/planfake/. It drives the low-level REST scan-planning client against independently authored wire payloads, but it does not itself plan — so it cannot yet power the epic's stated acceptance test.

Goal

Add the local-vs-remote parity test from the #1178 correctness contract: the same scan (same filter, same table, same snapshot) under scan-planning-mode=local and scan-planning-mode=remote must produce the identical FileScanTask set (paths, ranges, deletes, residuals).

Notes

• Needs a fixture that runs real planning server-side (e.g. the in-memory planner sketched in Epic: REST Catalog Scan Planning (server-side scan planning) client support #1178 Phase 6) so the remote path returns real results, or an equivalent that makes the comparison meaningful.
• This is the highest-risk validation in the epic — silent expression/wire-codec divergence is the failure mode — so it should cover schema-evolution and use-snapshot-schema edge cases.
• Depends on Phase 5 (scanner delegation) being wired.

[Revanth14]

@zeroshade Thanks for pointing this out, will look into this.