feat(query): aggregate and window answers independent of input order (#547, PR 1/3) - #642

PR 1 of 3 for #547 (part of #473): aggregate and window rules on the executor. Spec: docs/superpowers/specs/2026-10-20-aggregate-window-engine-design.md (#640). Plan: docs/superpowers/plans/2026-10-20-547-aggregate-window.md.

No aggregate or window answer depends on the order rows reach it any more.

Changes

• New src/query/datalog/aggregate.rs, shared by both engines (PRs 2–3 will use it):
  • value_order: one total order of values. Numbers compare exactly across Integer and Float, and NaN comes after every other number. After the numbers come strings, booleans, refs, keywords, and nil last.
  • Exact sums: integers add in an i128 and saturate once. Floats add into a fixed-point accumulator and round once, half-even.
  • Agg and Running: the accumulators for aggregates and windows. Type errors (INT-041, INT-043, INT-044) and dropped groups are decided once all rows are in, so the answer is the same whatever order the rows arrive in.
• Aggregation: one pass into BTreeMap<Vec<Value>, Vec<Agg>>; groups finish in key order.
• Windows:
  • partitions are keyed by Value's Ord, so all NaN keys share one partition;
  • rows sort by :order-by, then the other :find-named variables, then input position;
  • peers (rows that tie on the order key) share rank and the running value at the last peer, as in SQL.
• PRS-082 (new): a window in an aggregate query may read only plain :find variables.
• functions.rs: the built-in functions become an enum. AggState, WindowOps, apply_builtin_aggregate, value_lt, value_cmp, is_known and is_window_compatible are removed.

Changed existing tests

• tests/window_functions_test.rs::mixed_aggregate_and_window_in_same_find and parser.rs::parse_mixed_aggregate_and_window_allowed used exactly the query PRS-082 now rejects (a window reading a variable that is not grouped). Both now use a grouping variable. New tests check the rejection.

Tests

• New: 15 unit tests (aggregate/tests.rs), 8 integration tests (tests/aggregate_window_rules_test.rs), and 8 aggregate/window queries added to the plan-independence matrix.
• Totals: 1616 tests, 1606 passing and 10 ignored. test-oracle is green.
• Clippy (lib and --all-features) is clean, and so is the wasm browser build.

Not in this PR

• Deferred index-order work (Sort elision, sort-grouped Aggregate): perf(query): proved Sort omission, one-group count and partition windows #641.
• The docs-site PR follows.
• Spec §3.2 lists avg among the aggregates, but avg is window-only (PRS-027), so Agg has no avg. The plan records this deviation.

Codex review (read-only, on the commit) found no issues.

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