v0.3.0

• A when() stub and an expect() for the exact same call are refused at
  registration with the new ConflictingExpectation (#59).
  The two never composed: whichever was declared later took the dispatch, and
  the earlier one silently lost its purpose — the stub's answer replaced by
  the mode default, or the count starved and reported as "called never" about
  a call that did happen. Both orders now fail fast, naming the double, the
  specification and the one-registration idioms (expect(...)->returns(...),
  when(...)->times(...)). Dispatch semantics are untouched: two plain stubs
  still layer most-recent-first, and overlapping-but-different specifications
  (a broad fallback under a narrow claim) still compose. Registering over a
  counted stub (when(...)->times(...)) is refused the same way, for the
  same reason.