feat(interp): stage-level time-sample evaluator

[bresilla]

USD attributes can carry timeSamples but the crate has had no evaluator for them so far. Anyone wanting a value at a specific stage time had to read the raw samples and interpolate by hand. This PR adds the evaluator so consumers can just ask.

src/
├── interp.rs       # InterpolationType + evaluate()
└── stage.rs        # value_at(), getter/setter, builder method

tests/
└── interp.rs

fixtures/
└── interp_scene.usda

Stage::value_at(attr, time) matches C++ UsdAttribute::Get(time). It reads timeSamples, picks the bracketing pair, interpolates per the stage's mode, or returns the attribute's default when there are no samples.

The behaviour follows AOUSD §12.5:

• Linear is the default mode.
• Quaternions interpolate via slerp (lerp would take the wrong arc).
• Types not listed in §12.5.2 fall back to held.
• A ValueBlock in the bracketing pair returns None.
• Before the first sample and after the last, the value is clamped.

11 unit + 11 integration tests, all green. ROADMAP §12.5.1 and §12.5.2 flip to ✅.

Heads-up: a follow-up PR will add UsdSkelAnimQuery on top of this — a thin wrapper that pulls a SkelAnimation's translations / rotations / scales through value_at so the existing SkeletonResolver / SkinningResolver can be driven at a stage time.

[mxpv]

Nice!