rx build --trace: structured build tracing with suggestions

[iPeluwa]

Overview

Add a --trace flag that emits structured timing spans showing exactly where time is going during a build, with actionable suggestions.

Example output

rx build --trace

  ┌─ rx startup              12ms
  │  ├─ parse rx.toml         3ms
  │  ├─ detect linker         4ms
  │  └─ load workspace        5ms
  ├─ cache lookup             8ms  (miss)
  ├─ cargo build            18.3s
  │  ├─ compile mylib        6.1s  ← bottleneck
  │  ├─ compile myapp        4.2s
  │  └─ linking              8.0s  ← second bottleneck
  └─ cache store             120ms

  Suggestions:
  → mylib takes 6.1s — consider splitting into smaller crates
  → linking takes 8.0s — mold would reduce this to ~1.5s

Implementation

• Use cargo's --timings JSON output to extract per-crate durations
• Instrument rx's own overhead (config load, cache lookup, etc.)
• Identify bottleneck crates and linking time
• Suggest actionable fixes (mold, crate splitting, etc.)

Priority

v0.3 — diagnostic gold for large projects