docs: document the identity contract of the serializer constant labels

[velo]

Documentation only, no behaviour change.

SerializerBase.constantToLabel is an IdentityHashMap keyed on arbitrary
constants supplied by the caller, and getConstantToLabel() exposes it as public
API. Nothing said so, which matters in two directions:

• A dialect that overrides serializeConstant to bind by label depends on
  the identity keying; one that binds positionally (JPA and SQL both do) does not.
• It is a deliberate identity dependency on user values. JEP 401 (Value Objects,
  preview in JDK 28) migrates the primitive wrappers and LocalDate to value
  classes, at which point == and identityHashCode become state-based and the
  map starts collapsing equal-but-distinct constants onto one label.

Added javadoc on the field and the accessor recording both, and pointed at
SerializerBaseTest, which pins the current behaviour.

🤖 Generated with Claude Code

https://claude.ai/code/session_01WnNBe1GoG2SxfU3FN7bcAA