[29%] Integrate critter-core into morphia-core

[evanchooly]

Goal

Integrate critter-core into morphia-core to provide a modern Mapper architecture using bytecode generation, replacing legacy reflection-based implementation. Users get a config-driven choice between mappers with automatic fallback.

Implementation Phases

• [100%] Phase 1: Extract Mapper interface and add config option #4184
• [100%] Phase 2: VarHandle accessor generator #4185
• [50%] Phase 3: Convert Kotlin to Java #4195
• [0%] Phase 4: Move critter-core into morphia-core #4186
• [0%] Phase 5: CritterMapper implementation #4187
• [0%] Phase 6: Wire CritterMapper into MorphiaDatastore #4188
• [0%] Phase 7: Test infrastructure and CI matrix #4189
• [0%] Phase 8: Cleanup and documentation #4190

Planning Document

Full implementation plan with code samples, file references, verification steps, and risk mitigation:
docs/critter-integration-plan.md

Design Decisions

Decision	Choice
Generation strategy	Hybrid: check classpath for pre-generated models (critter-maven AOT) → runtime generation (Gizmo + VarHandle) → reflection fallback
Runtime accessors	VarHandle-based — no synthetic __read/__write methods needed at runtime
Language	Convert to Java — convert critter Kotlin source before moving into morphia-core
Mapper interface	@MorphiaInternal only — not exposed on any public API
Fallback granularity	Entity-type level — all-or-nothing per type
copy() semantics	Share immutable CritterEntityModel references
Dependencies	Optional + fail-fast — Gizmo/ASM/ByteBuddy marked <optional>, clear error if missing when CRITTER selected
Classloader	Uses configured classloader as parent; build-time classes are permanent, runtime classes regenerated per JVM start
Module removal	Last step — only after full test matrix is green