Replace cereal snapshots with sealed component JSON serialization

[BenjaFriend]

Summary

• Replace cereal world snapshots with a sealed ComponentTypeRegistry and entity-centric JSON so levels are human-readable and Gameplay no longer lists Graphics types (unblocks the issue 169 module split).
• Load Transform first, iterate meshes with EnTT views instead of owning groups, and list named entities in the world outline so JSON levels and Play-spawned grids keep their authored pose.
• Related: issue 171 (null-check MeshRenderer model/material on save).

Implementation

• Fling::Json (PIMPL) wraps nlohmann privately. Public headers no longer include <nlohmann/json.hpp>.
• JsonArchive is a single bidirectional Serialize (Unreal FArchive style) with MakeNVP / FLING_FIELD.
• ComponentTypeRegistry: explicit Register<T> then Seal(). LoadLevelFile / OutputLevelFile require sealed. No FLING_COMPONENT static auto-registration.
• Startup: Engine registers gameplay + graphics components, World::Init runs Game::RegisterComponents(), external registrars, Game::Init(), Seal(), then loads StartLevel.
• Renderer uses view<Transform, MeshRenderer, tag> so assign<Transform>() references stay valid. World still loads Transform before other JSON keys so PointLight / mesh hooks see the authored pose.

Test plan

• FlingTests — 172 assertions in 17 test cases, all passed (JSON round-trip, registry Seal, unknown keys skipped, distinct Transform positions, view vs owning-group assign)
• Launch Sandbox, confirm Level_1 entities (Floor / Sun / Sphere) appear in the world outline with names
• Press Play and confirm the LightingTest grid is spaced out, not stacked at the origin
• Save/load a level and confirm git-diffable JSON

[BenjaFriend]

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