Stratified negation, aggregates, and the maintainability check at registration (V5) #312

Part of epic #305.

Problem. Negation is a side computation that collects right-hand keys into a HashSet, runs nested execute_directly for complex right sides, silently skips live-only relations and returns an empty set on panic (fails open) (src/code_generator/mod.rs:1951-2075); static prefilters assume complete input (static_input.rs:120-180); aggregates are DD reduce but only inside per-query dataflows (:2279-2521). Constructs that cannot be maintained (now()/time decay :2933,2964-2976, non-deterministic functions) are accepted in persistent rules today; vector predicates (hnsw_nearest resolved outside the dataflow, src/execution/hnsw_resolve.rs) are accepted and force full re-evaluation of the view on every commit.

Target. Negation via DD antijoin over the lower stratum's arrangement (unstratifiable programs already refused at registration, src/rule_catalog.rs:259-296); aggregates count/sum/min/max/avg via reduce, group by; a maintainability check at +rule registration refusing now(), time decay, non-deterministic functions and sort/limit annotations in persistent rules with an error naming the construct and the alternative (query or session rule) (captain D3). Vector predicates and functions (hnsw_nearest, within_radius, distance and LSH functions) are NOT refused: until VV4 and VV5 land, a rule using them is served by the recompute path behind the flag and marked views_pending{reason=vector}; the default flip (V20) waits for VV1-VV7. Rules loaded from disk that fail the check are reported at load and served by recompute until fixed (tracked in V20's fleet scan).

Acceptance. Oracle: negated insert/delete, recursive upstream settling, zero/multiple supports, float sums, min/max replacement, empty groups; no transient false rows observable at any revision; registration error messages tested; a catalog containing refused constructs loads with a warning and a views_unmaintainable{rule} metric.

Size L. Dependencies V4. Order slot 3. Supersedes #183, #184.