Cache frozen FieldType to skip schema validation (#15886)

When indexing, every field in every document has its schema built via
updateDocFieldSchema and validated via assertSameSchema against the
existing FieldInfo. For the common case where a field consistently uses
the same frozen FieldType instance, this work is redundant — a frozen
type is immutable, so its schema contribution is identical every time.

This change caches the frozen FieldType on each PerField and checks
same object instance to detect when the type hasn't changed. When it
matches, schema building and validation are skipped entirely, and
FieldSchema only resets its docID. If a different type is encountered,
the cache is invalidated and the full validation path runs. A
deoptimize path handles multi-valued fields where a later value uses a
different type than earlier values within the same document.

Adds FieldType.isFrozen() to support the optimization, and new tests
covering the fast-path, cache invalidation, deoptimize, cross-segment
validation, and document blocks.