quack: refuse an empty group universe in lower_group_avg; record lancedb remote cost

[AdaWorldAPI]

Follow-up to #1282, which merged before these two commits.

lower_group_avg refuses groups == 0 (codex P2 on #1282)

Codex flagged the new Pair arm. The problem is wider: the count half of every AVG plan is GroupReduce Count, and that terminal needs a non-empty Out::I64. So a K = 0 plan could never execute for Local and Via keys either, and the zero case was accepted for all of them. The function now returns LowerError::EmptyGroupUniverse for every key kind, the same as lower_group_having.

• New test lower_group_avg_refuses_an_empty_group_universe_for_every_key_kind. It covers Local and Pair keys, and asserts both that K = 0 is refused and that K = 1 lowers.
• The test went red before the fix (Local(Col(1)): Ok where Err was expected). A disable run that deletes the guard turns it red again.
• quack: 28 lib + 39 integration tests green; clippy -D warnings clean.

The remote feature's cost, recorded (stranded from #1282)

crates/lance-graph/Cargo.toml and the sweep's board entry now record the measured cost of lancedb's remote feature. Measured with cargo tree -p lance-graph --features lancedb-sdk, it adds 10 crates (630 vs 620):

• New: arrow-flight, axum 0.7 and 0.8 (two majors), axum-core ×2, matchit ×2, tower-http, serde_path_to_error, httpdate.
• Already present via lance: reqwest, tonic, prost, http.

This part is a comment and documentation only; no behaviour changes.

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• crates/lance-graph-quack/src/lib.rs
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