Vector columns in maintained views (VV1) #323

Part of epic #305.

Problem. Vectors are Value::Vector(Arc<Vec<f32>>) (src/value/mod.rs:222) and distance/LSH functions are pure (src/vector_ops.rs), but no maintained view exists to carry them, there is no centroid aggregate, and nothing proves that vector-carrying views stay exact under embedding updates.

Target. Views carry vector columns through arrangements by shared pointer (one copy per graph per distinct vector); pure vector functions (cosine, dot, euclidean, int8 variants, normalize, scale/add, lsh_bucket, lsh_probes) allowed in deployed rules and compiled into the maintainer like any function; a vec_mean aggregate (centroid of a group) maintained by reduce; an embedding update is a row retraction plus insertion through every view.

Acceptance. Oracle on random histories with embedding updates: vector-carrying views equal recompute at every revision; memory test: two views carrying 100k vectors add <= 32 B per row over the base store; vec_mean retraction correctness.

Size M. Dependencies V3. Order slot 7.