Future: Advanced gravity features (per-joint magnitude, time-varying, zones)

## Description

Potential future enhancements to the SpringBone gravity physics system. These are post-MVP features for advanced use cases.

## 1. Per-Joint Gravity Magnitude

Allow each joint to have independent gravity magnitude, not just scaled from global gravity.

**Current:**
```metal
float3 effectiveGravity = globalParams.gravity * boneParams[id].gravityPower * boneParams[id].gravityDir;
```

**Proposed:**
```swift
struct BoneParams {
// ...
float gravityPower; // Magnitude multiplier
float3 gravityDir; // Direction (normalized)
float gravityMagnitude; // Optional: override global magnitude
}
```

**Use Case:** Hair on the moon (low gravity) while character is on Earth

## 2. Time-Varying Gravity

Support animated/keyframed gravity for special effects.

**API Concept:**
```swift
// Gravity changes over time
renderer.setGravity(
SIMD3(0, -9.8, 0),
animationCurve: .sine,
duration: 2.0
)
```

**Use Cases:**
- Antigravity effects
- Underwater simulation
- Zero-G sequences
- Magic spells

## 3. Gravity Zones

Define spatial regions with different gravity properties.

**Concept:**
```swift
struct GravityZone {
var bounds: BoundingBox
var gravity: SIMD3
var falloff: Float // Blend distance
}
```

**Use Cases:**
- Wind zones (horizontal gravity)
- Water bodies (buoyancy)
- Magical areas (custom physics)
- Multi-floor buildings (gravity direction changes)

## Implementation Complexity

- **Per-Joint Magnitude**: Low (shader + struct change)
- **Time-Varying**: Medium (animation system integration)
- **Gravity Zones**: High (spatial partitioning, zone detection)

## Priority

Low - These are advanced features beyond VRM 1.0 specification

## Related

PR #51 - Implement per-joint gravity for cloth and hair physics
Issue #52 - Enhance gravity formula

Start by reviewing PR #51 and issue #52, which are the related work identified in the issue. The proposal covers three separate features with different implementation complexity, so a contributor would first need a maintainer decision on which feature to scope; done would mean implementing and validating that selected feature.

Written by the indexing model from the issue text.