Portable assertion accounting for the moonpool framework.

This is the Antithesis-style assertion suite — boolean (always / sometimes / reachable / unreachable), numeric guidance with watermarks, compound sometimes-all with frontier tracking, and per-value sometimes_each buckets — factored out of moonpool-explorer so it has zero dependencies and compiles on wasm32-unknown-unknown, macOS, and Linux.

How it relates to exploration

The counts live in a region of memory. By default [init] allocates that region on the heap (single process — wasm, macOS, plain native test runs). moonpool-explorer instead allocates a MAP_SHARED region and hands it to [install_region], so the same accounting is visible across forked children, and installs a [DiscoveryHooks] that turns "a new assertion state was reached" into coverage marking and fork dispatch. With no hooks installed the accounting is pure: assertions still record pass/fail/watermark/frontier, which is all the UntilCoverageStable stop condition and contract validation need.

See [hooks] for the coupling surface and [region] for the storage model.