Add TagMap read-through support (level-split phase 1 mechanism)

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 <noreply@anthropic.com>

Author: dougqh