Post-fold tidy, all TagMap-scoped:

- Remove EmptyHolder: with one class there is no interface<->impl class-init
  cycle to break, and the private constructor reads no statics, so EMPTY is a
  direct `new TagMap(new Object[1], 0)` initializer.
- Static factories (create/fromMap/ledger/...) are now `public static final`
  (not expressible on the old interface).
- assertSize/assertNotEmpty/assertEmpty/checkIntegrity test helpers dropped
  their now-always-true `instanceof TagMap` guard + redundant cast.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

The interface -> final-class fold removed the TagMap interface decls, which
carried the @Nullable/@nonnull param annotations from #11963. Re-home them
onto the now-concrete methods: @nonnull tag keys and strict-setter values,
@nullable on the set(EntryReader)/getAndSet(Entry) sinks (+ the getAndSet
contract javadoc). The null-tolerance behavior was already preserved by the
fold; this restores the self-describing contract on the write surface.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

Per review: these factories NPE on a null map (map.size()/putAll), so the
input is non-null by contract.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

A fresh, mutable TagMap can read through to a frozen parent on local
misses, so a span can layer its own tags over a shared, immutable set
(e.g. merged tracer tags) without copying them.

- createFromParent(parent): the only way to attach a parent; the parent
  must be frozen and is fixed at construction (no re-parenting), so
  read-through can treat it as stable. Single-parent by design in phase 1.
- Reads resolve local-first, then the parent; a local entry shadows the
  parent's (local-wins). Removing a parent key locally records a lazy
  tombstone (removedFromParent) so it stops reading through; the tombstone
  set is null until first needed, keeping the hot paths untouched.
- size()/isEmpty() are exact (Map contract) and resolve the parent;
  isDefinitelyEmpty()/estimateSize() are the cheap conservative variants
  for the hot path. copy() preserves the parent and tombstones; forEach
  walks local then parent.

Built on the folded final-class TagMap (#11967); composes cleanly with the
null-tolerant Entry pathway (#11963).

Co-Authored-By: Claude Opus 4.8 <[email protected]>