Export Uzu traces as safetensors

[ry2009]

Summary

Reworks the Uzu tracing integration around activation/tokenization export instead of local comparison:

• removes the old trace comparator/metrics implementation from the Uzu integration tracer
• exports Uzu activation traces to traces.safetensors-compatible safetensors output
• exports Uzu tokenization traces with activation_trace.token_ids / activation_trace.token_positions plus rendered request metadata
• keeps trace input loading strict for token id/position shape
• adds a minimal safetensors writer for exported trace tensors and metadata
• keeps the reusable writer byte/schema oriented instead of tying it to backend arrays

This pairs with the Lalamo comparator PR: trymirai/lalamo#285

Validation

• cargo test -p backend-uzu test_safetensors_metadata_writer --test integration
• cargo check -p backend-uzu --tests
• cargo fmt --check -p backend-uzu
• git diff --check

cargo fmt --check passes but prints existing stable-rustfmt warnings about nightly-only config keys.