0.2.0 First Leg

Lifetime: the store now forgets, on purpose and on Relay's terms.

• Retained roots. A @Query storage retains its handle while the view lives
  and releases it when SwiftUI drops the view's state. Released handles wait
  in a release buffer (Environment.releaseBufferSize, default 10, oldest
  out first); retained and buffered handles are the roots that keep records
  alive.
• Collection. Mark and sweep over the roots' plans, coalesced into one pass
  per batch of releases; swept records are cleared so cycles break, and the
  root's links to them are dropped. Environment.collect() runs it on demand.
• Fetch policies, as @Query("…", fetchPolicy:): storeOrNetwork,
  storeAndNetwork (the default), networkOnly, storeOnly. A storeOnly
  operation without data fails with MissingDataError.
• Staleness. Environment.invalidate() marks everything stale and refetches
  retained handles while their data stays visible; queryCacheExpiration
  does the same by age; isStale on every operation value.
• Preload parks a fetching handle in the buffer; a view attaching within the
  window finds the request in flight or the data present. Equal operation
  values share one handle and never fetch twice at once; settle() awaits
  the fetch in flight.
• RecordedTransport takes a responder closure, for tests and benchmarks that
  serve many pages.
• The compiler's scanner skips attribute arguments after the GraphQL literal.

Tests: re-entry within the buffer makes no request and eviction does;
collection removes unreachable records and keeps shared ones; each policy's
first-attach behaviour; invalidation and expiration refetch; preload. The
bench scrolls 42 synthetic pages through a buffer of 10 and reports records,
roots and footprint per page, plus the cost of a collection pass.

Deliberately not added: field-level invalidation, off-main marking, a
collection budget per pass, holdGC for optimistic updates (0.3).

Numbers for the scroll bench are in BENCHMARKS.md.