Improve architecture and performance

[Garume]

Summary

• Refactors scene spawning so Core no longer depends on Aquarium-specific actor creation.
• Adds focused EditMode tests for scene lifecycle, settings normalization, receiver behavior, boid edge cases, jellyfish geometry, and performance allocation checks.
• Optimizes update and draw hot paths toward zero-allocation execution.
• Documents the design review findings and performance benchmark results.

Key Changes

• Scene-side spawning replaces actor-owned instantiation.
• TargetTrackingNode now treats (0, 0) as a valid target instead of using it as a sentinel.
• Receiver data uses value-type payloads instead of per-detection objects and delegates.
• AquariumScene maintains typed indexes for food and shockwave actors.
• Jellyfish head geometry now uses consistent edge point calculations for fill and frame paths.
• Unused node GUID identity and IInstantiable were removed.

Validation

• Unity batchmode EditMode tests: 14 passed / 0 failed.
• Headless benchmark completed successfully.
• Benchmark result: 0.17963785 ms/frame, 0 allocated bytes in the measured update section.
• git diff --cached --check passed before commit.

Notes

Unity generated local Packages/ and ProjectSettings/ migration files during validation. Those files were intentionally left out of this PR because they are unrelated to the architecture and performance changes.