v0.2.0.1 Availability Performance

This release productionizes the availability-performance work: a prepared point-in-time provider, typed diagnostic blocks, linear event buffers, prepared fold-time valuation, linear seal validators, and atomic resumed-run equity finalization. It also corrects the peer-benchmark boundary so memory-backed rows use public sweep workflows. Compatibility: ledgr now requires collapse 2.1.8 or later; there is no 2.1.7 fallback. No public API, schema, hash, run-identity, accounting-policy, or availability-policy contract changes. On the accepted full fixture, the availability path measured 119.11 seconds cold and a 17.55-second warm median; the corrected public canonical and compiled memory sweeps measured 30.96 and 16.63 seconds, while durable ledgr measured 58.23 seconds. These are fixture-specific records, not universal speed claims. Windows and Ubuntu checks, pkgdown, and coverage passed on the release branch, main, and the release tag. Full details are in NEWS.md and the v0.2.0.1 release closeout.