Pin ref-slot diff wire states and make Reference.value final

[knutwannheden]

Motivation

• Follow-up to Never CHANGE a ref-deduplicated RPC slot; re-ADD the new value #8392, which fixed the Reference.asRef flyweight and settled the re-ADD semantics for changed ref-deduplicated slots. The DELETE, identical-value NO_CHANGE, and class-changed ADD paths of a ref-wrapped slot diff were equally corrupted by the old flyweight (both sides of the comparison collapsed onto one mutable wrapper) but have no wire-level regression coverage on main. This adds it, and hardens Reference so the per-call-instance guarantee that Never CHANGE a ref-deduplicated RPC slot; re-ADD the new value #8392's semantics rely on is structural rather than comment-enforced.

Summary

• RpcSendQueueTest: three new tests pinning the wire states a diffed ref slot emits — deletedRefSlotIsSent (CHANGE, DELETE), identicalRefSlotIsNoChange (CHANGE, NO_CHANGE), changedRefSlotOfDifferentClassIsAdded (CHANGE, ADD) — including that the onChange consumer sees the after value (or is not invoked for DELETE/NO_CHANGE), via a small diffRefSlot helper that seeds the queue's before context the same way senders do
• Reference.value is now final, assigned through a private constructor; a protected no-arg constructor keeps the GitProvenance subclass compiling unchanged

Test plan

• ./gradlew :rewrite-core:test --tests "org.openrewrite.rpc.RpcSendQueueTest"
• Full ./gradlew :rewrite-core:test (covers GitProvenance marker round trips against the constructor change)