feat: v0.6.0 — observer / stop-conditions / tracing / IGD / R2

Theme: production lifecycle. heuropt becomes deployable for long-
running, real-world workloads. No breaking changes — Optimizer trait
gains a default-impl run_with method that falls back to run.

Adds:
- src/observer/ module: Snapshot, Observer trait, ControlFlow, plus
  built-in MaxTime / MaxIterations / TargetFitness / Stagnation /
  Periodic / AnyOf / AllOf and a closure impl.
- Optimizer::run_with(problem, observer): default-impl on the trait,
  overridden for full per-gen visibility on Nsga2, RandomSearch, and
  DifferentialEvolution. Other algorithms inherit a final-only
  notification — full per-gen support follows incrementally.
- New 'tracing' optional feature plus TracingObserver that emits
  structured debug! events per generation.
- src/metrics/igd.rs: IGD + IGD+ performance indicators against a
  reference set.
- src/metrics/r2.rs: R2 indicator using the weighted Tchebycheff
  utility; pair with das_dennis for the canonical weight set.
- examples/constrained.rs: BNH constrained 2-objective problem
  solved with NSGA-II + observer composition (MaxTime.or(Periodic)).

Bumps Cargo.toml to 0.6.0; CHANGELOG entry consolidates the above.
Existing 247 unit + 38 doctest + 32 algorithm-property + property /
metric / numerical-stability tests all pass; bit-identical compare
output verified post-DE refactor.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "heuropt"
-version = "0.5.0"
+version = "0.6.0"
 edition = "2024"
 rust-version = "1.85"
 authors = ["Stephen Waits <steve@waits.net>"]
@@ -17,12 +17,14 @@ categories = ["algorithms", "science", "mathematics", "simulation"]
 default = []
 serde = ["dep:serde"]
 parallel = ["dep:rayon"]
+tracing = ["dep:tracing"]
 
 [dependencies]
 rand = "0.9"
 rand_distr = "0.5"
 rayon = { version = "1", optional = true }
 serde = { version = "1", features = ["derive"], optional = true }
+tracing = { version = "0.1", optional = true, default-features = false, features = ["std", "attributes"] }
 
 [dev-dependencies]
 gungraun = "0.18"