v0.5.435-rete-plus-develop: β-A skip counters + measurement

Builds on v0.5.434 with one telemetry change:

- 7c308399b perf(rete): β-A skip counters + benchmark output of gate
  effectiveness

Adds two SelectiveEvaluator counters (total_relational_alpha_skips_-
disjoint, total_relational_alpha_skips_source_type) so we can see how
much the β-A watched-property gate and the source-type filter actually
save on real recipes. Surfaced via get_rete_stats() and printed in the
two_origins benchmark output as a "gate effectiveness" ratio.

Initial measurement (local, two_origins):
  Relational alpha evals:        21,068
  Rel. skips (disjoint):          4,304
  Rel. skips (src-type):            483
  Rel. gate effectiveness:        18.5%

This closes the β-A measurement question (Phase 2 ship-or-revert
decision = SHIP, already shipped via aca0c576a / 5ad7d4a57 / the
network_builder.relational_alpha_watched wiring; just wasn't visible
in stats output until now).

Tasks #128 (alpha-by-node-type index) and #185 (cluster 2.4x gap)
also closed in this turn — superseded by the measurement here and the
cluster-noise finding respectively.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>