bench: add legacy-vs-new comparison benchmarks for SimpleMapStore

Rewrites simple_store_bench.rs to benchmark all four store variants
(LegacyPerKey, LegacyGlobal, NewPerKey, NewGlobal) across insert,
range query, exact query, store-analyze, concurrent reads/writes,
mixed read-write, cleanup overhead, query patterns, label cardinality,
and multi-agg-id scenarios.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: zz_y