spotbugs: suppress 6 design-intentional cast/CT findings

[Vest]

Add method/constructor-level @SuppressFBWarnings with one-line justifications for casts and constructor-throws that are intentional by design.

Suppressions

#	Finding	Location	Justification
1	BC_UNCONFIRMED_CAST	ConcretePersistentTransitionChoice.setChoiceActor	API contract documents @throws ClassCastException for non-PersistentChoiceActor; the cast is the contract
2	BC_UNCONFIRMED_CAST	FactGrouping.process	Only CDOMObject can have facts (per in-code comment); type system can't express this constraint as PCGenScoped is the wider interface
3	BC_UNCONFIRMED_CAST_OF_RETURN_VALUE	LevelInfoFacet.get	Facet's own cache; getComponentSet() always returns ArrayList<PCLevelInfo>
4	BC_UNCONFIRMED_CAST_OF_RETURN_VALUE	UnencumberedArmorFacet.getBestLoad	Facet's own cache; getComponentMap() always returns TreeMap<Load, Set<Object>>
5	BC_UNCONFIRMED_CAST_OF_RETURN_VALUE	UnencumberedLoadFacet.getBestLoad	Facet's own cache; getComponentMap() always returns TreeMap<Load, Set<Object>>
6	CT_CONSTRUCTOR_THROW	AbstractReferenceManufacturer constructor	Abstract class; ManufacturableFactory is required and Objects.requireNonNull is the lightest defense against an unusable instance

SpotBugs delta (XML-verified)

• BC_UNCONFIRMED_CAST: 3 -> 1 (-2; the third occurrence in VariableUtilities is handled in a separate PR)
• BC_UNCONFIRMED_CAST_OF_RETURN_VALUE: 3 -> 0 (-3)
• CT_CONSTRUCTOR_THROW: 29 -> 28 (-1)
• Total: -6, no new findings

Validation

• ./gradlew compileJava -> BUILD SUCCESSFUL
• ./gradlew :test --tests "pcgen.cdom.*" -> 2355 tests, 0 failures