Implement InferenceImplementationProviders (Float32, SymInt32, Asym)

## Context

Sub-issue of #58.

**Blocked by:** #59 (scaffolding)
**Blocks:** #62 (ir2c), #64 (E2E tests)

## Goal

Implement `InferenceImplementationProvider` protocol for all three quantization types.

## Protocol (defined by creator)

```python
class InferenceImplementationProvider(Protocol):
def model_attributes(self, sub_graph: DataGraph) -> AttributeMapping: ...
```

Simpler than training — only `model_attributes()` with forward-pass C symbols.

## Acceptance Criteria

- [ ] `Float32InferenceProvider` implemented and tested
- [ ] `SymInt32InferenceProvider` implemented and tested
- [ ] `AsymInferenceProvider` implemented and tested
- [ ] Providers return valid attribute mappings accepted by `apply_inference_provider`
- [ ] C symbol names match actual function names in the C framework