GEP-27: reconcile with implementation — @PackedClosures name and mode diagnostics

Align the doc with what shipped:
- rename @Packed -> @PackedClosures throughout (14 occurrences); the annotation is
  named for what it packs (closures), and since a dynamic arrow-lambda is itself a
  closure with no separate dynamic-lambda packing, the closures-specific name is
  exact -- note added;
- replace the boolean @Packed(strict = true) design with the delivered
  @PackedClosures(mode = LENIENT | WARN | STRICT) enum (silent / compiler warning
  per decline with reason / compile error), and state that the automatic
  groovy.target.closure.pack path is always lenient;
- drop 'designed but not yet wired' -- the mode diagnostics are implemented and
  listed among the delivered safety mechanisms.