v0.5.2 — bench harness expanded to three comparators

Changed (docs)

• benchmarks.md per-operation results now compare ART against three
  implementations rather than one. The bench/ harness gained _Tidwall
  (github.com/tidwall/btree v1.8.1, configured with
  Options{NoLocks: true, Degree: 32} for parity with google/btree's default
  degree and to keep its sync.RWMutex out of the measurement) and _Plar
  (github.com/plar/go-adaptive-radix-tree v1.0.7, whose public API stores
  interface{} values — the per-Put boxing alloc that costs is structural to
  the API and is documented in-table) siblings of Put, Get, GetMiss,
  Delete, and Range. The Verdict prose has been rewritten to reflect the
  new three-comparator landscape rather than the prior google/btree-only
  framing. No core .go files were edited; the change is confined to the
  nested bench/ module and the benchmarks.md doc.