v0.5.1 — refactor + CI/test fixes

Changed (internal)

• Deduplicated the four-way prefix-consume / terminal-check preambles in
  get.go, delete.go, and iterate.go behind three free helpers in
  helpers.go (consumePrefix, terminalValue[V], yieldTerminalInRange[V]).
  −40 net LOC. No additions to the node / innerNode interfaces; every
  helper is a free function over concrete types so no interface-method
  indirection is introduced (runtime.getitab remains absent from Get and
  Delete pprof profiles). put.go is intentionally left expanded — sharing
  a helper across the prefix-split allocation boundary would reintroduce
  interface dispatch on the hot path, per the reverted polymorphism spike
  recorded in polymorphism-failed.md. Hot-path benchmarks at parity with
  v0.5.0 within run variance.

Fixed

• artmap/ordered_test.go: the TestOrdered_Float64_OrderAcrossZero fixture
  now uses math.Copysign(0, -1) for actual IEEE-754 negative zero. The
  prior -0.0 literal was folded to +0.0 by the parser (staticcheck
  SA4026); the test now exercises the intended sign-bit case.

CI

• The fuzz-smoke workflow step now discovers Fuzz* targets dynamically
  across all packages and runs each one scoped to its own package. Replaces
  a go test -fuzz=… ./... invocation that was rejected by Go with
  "cannot use -fuzz flag with multiple packages" once the artmap
  subpackage was introduced. Future fuzz targets in any package are picked
  up automatically; an explicit empty-set guard fails the step if no
  targets are discovered.