feat(ecs): add FieldType, FieldDescriptor, ComponentMeta reflection types

[JeanPhilippeKernel]

Dependency position

THIS ISSUE ──► ComponentReflectionRegistry (#649)
            └──► Register built-in components (#650)
(no dependencies — start immediately, parallel with #647)

This is the start of Track B. No dependencies. Work it in parallel with #647.

---

Why

These three data structures are the vocabulary of the reflection system. Every other issue in this track depends on them. They are pure type definitions — no logic, no external dependencies beyond <cstdint>.

Work

Create two new header files:

ZEngine/ZEngine/ECS/Reflection/FieldType.h

#pragma once
#include <cstdint>

namespace ZEngine::ECS {

    enum class FieldType : uint8_t {
        Bool,
        Int8, Int16, Int32, Int64,
        UInt8, UInt16, UInt32, UInt64,
        Float, Double,
        Vec2f, Vec3f, Vec4f,
        Quatf,
        Mat4f,
        EntityID,   // display as "Entity #N (gen M)"
        AssetUUID,  // look up asset name via AssetRegistry
        String,     // fixed char[N] buffer
        Enum,       // dropdown; requires EnumValues array
        Struct,     // nested sub-fields
    };

}

ZEngine/ZEngine/ECS/Reflection/ComponentMeta.h

#pragma once
#include <ZEngine/ECS/Reflection/FieldType.h>
#include <ZEngine/ECS/ComponentTypeID.h>
#include <cstdint>

namespace ZEngine::ECS {

    struct EnumValue {
        const char* Name  = nullptr;
        int64_t     Value = 0;
    };

    struct FieldDescriptor {
        const char*            Name        = nullptr;
        FieldType              Type        = FieldType::Float;
        uint32_t               Offset      = 0;        // offsetof(Component, field)
        uint32_t               Size        = 0;        // sizeof(field)
        float                  Min         = -1e9f;
        float                  Max         =  1e9f;
        uint32_t               StringCap   = 0;        // for FieldType::String
        const EnumValue*       EnumValues  = nullptr;
        uint32_t               EnumCount   = 0;
        const FieldDescriptor* SubFields   = nullptr;  // for FieldType::Struct
        uint32_t               SubCount    = 0;
        bool                   Hidden      = false;
        bool                   ReadOnly    = false;
        const char*            Tooltip     = nullptr;
    };

    struct ComponentMeta {
        ComponentTypeID        TypeID      = 0;
        const char*            TypeName    = nullptr;
        uint32_t               Size        = 0;
        uint32_t               Align       = 0;
        const FieldDescriptor* Fields      = nullptr;
        uint32_t               FieldCount  = 0;
        const char*            Category    = "General";
        const char*            Tooltip     = nullptr;
    };

}

Acceptance criteria

• Both headers compile cleanly with no external dependencies beyond ComponentTypeID.h
• No .cpp file needed — these are pure data structures
• FieldDescriptor::Offset and Size are populated using offsetof() and sizeof() at the registration call sites (not here)