[100%] Phase 5: CritterMapper implementation

[evanchooly]

Parent: #4179
Blocked by: #4186

Goal

Implement the hybrid discovery mapper with three-tier fallback: pre-generated → runtime generation → reflection.

Details

See Phase 5 in planning doc

Tasks

• Create CritterMapper extends AbstractMapper
• Implement fail-fast dependency check in constructor
• Implement tryLoadPregenerated() — load by naming convention from app classloader
• Implement tryRuntimeGeneration() — Gizmo + VarHandle via CritterClassLoader
• Implement fallbackToReflection() — standard EntityModel creation
• Implement per-entity-type error logging (once per type, not per access)
• Implement copy() — share immutable CritterEntityModel refs, new DiscriminatorLookup
• Thread safety: check-then-register pattern, synchronized classloader ops
• Tests: pre-generated loading, runtime generation, reflection fallback, concurrent access, copy semantics

Progress Tracking

phase_percentage = (completed_tasks / 9) * 100, rounded to nearest integer
gh issue edit 4187 --repo MorphiaOrg/morphia --title "[XX%] Phase 5: CritterMapper implementation"

parent_percentage = (total_completed_tasks_across_all_phases / 56) * 100, rounded to nearest integer
gh issue edit 4179 --repo MorphiaOrg/morphia --title "[XX%] Integrate critter-core into morphia-core"

When all tasks are complete (100%), close this issue and check its box in #4179.