v6: Phase 1 — LocalMock serves Arrow IPC + docs snippet + changelog note (server-gated)

[Pandys]

⛔ Gated on server Arrow support shipping (see #49). These are the remaining Definition-of-Done items for Phase 1.

Scope

• LocalMock (DESIGN.md §8) can serve Arrow IPC responses, so tests and examples/ exercise the Arrow path with no infrastructure. Ties into v6: clappform.testing.LocalMock — transport double #44 (LocalMock) and v6: unit test harness — in-process gRPC fakes #45 (gRPC fakes).
• Docs guide snippet showing df = cf.data.query("...").read() transparently using Arrow when clappform[arrow] is installed, plus an explicit .fetch().to_polars() example. Runs against LocalMock in CI (DESIGN.md §9). Ties into v6: docs — MkDocs Material on Read the Docs, generated reference, CI-tested snippets #48 (docs).
• Changelog + reference note that Arrow requires a compatible server/cluster.

Definition of done (the whole Phase 1, tracked here)

• to_arrow()/to_polars()/to_pandas() (Arrow-backed)/__arrow_c_stream__ behind [arrow]/[polars] with clear missing-dep errors (v6: Phase 1 — Arrow IPC consumption path on ReadResult (server-gated) #49)
• Transparent negotiation + correct JSON fallback for non-Arrow clusters (v6: Phase 1 — Arrow format negotiation + graceful JSON fallback (server-gated) #60)
• LocalMock serves Arrow IPC; tests + examples exercise it
• Docs snippet (transparent read + explicit Polars), CI-tested against LocalMock
• Changelog + reference note re: server/cluster requirement

Depends on: #49, #60. Related: #44, #45, #48.

Source: planning/pypi-arrow-handoff.md §6.