renacer-core: Zero-dependency tracing primitives

Extracted from renacer to break the renacer→aprender→realizr→renacer circular dependency (PMAT-284). This crate provides:

• [SpanRecord] — Parquet-compatible span schema (W3C Trace Context)
• [LazySpan] — Deferred span construction (zero overhead when unused)
• [SpanPool] — Memory pool for span allocations
• [TraceContext] / [LamportClock] — W3C trace context + causal ordering

Design

This crate has NO dependencies on aprender, realizr, or trueno. It can be used by any crate in the stack for uniform instrumentation.

Usage in realizr

use renacer_core::LazySpan;

let span = LazySpan::new()
    .with_name_static("decode_step")
    .with_attribute_static("m", state.m.to_string())
    .timed();  // starts timing
// ... GPU work ...
span.finish(); // records duration