Core: Precompute sorted entries in ShreddedObject SerializationState

[nssalian]

Changes

Precomputes a sorted Entry[] once in the SerializationState constructor.
writeTo becomes an index loop over the array.

Per-writeTo, this removes:

• SortedMerge.of(...) + two stream().sorted() iterators
• 2N HashMap lookups (shreddedFields.get + unshreddedFields.get)
• The Maps.newHashMap(shredded) defensive copy

The Maps.newHashMap(shredded) copy was there to stop the old code from accidentally
modifying the caller's map (#15087). The refactor never modifies the caller's map, so the copy is no longer needed.

Tests

Three new tests in TestShreddedObject:

• testPutAfterRemoveIsSerialized - remove(x) then put(x, v) semantics
• testEmptyObject - zero-field state
• testRepeatedWriteToProducesSameBytes - cached-state reuse across writes
• Ran additional diff tests locally from the main branch to ensure correctness.