docs: add README and crate-level //! docs

Adds:
- README.md following spec §19.1 (what / install / define problem /
  run NSGA-II / custom optimizer / current algorithms / design
  philosophy).
- A short-but-runnable crate-level //! example in lib.rs for
  `cargo doc` (spec §19.2).

README.md

@@ -0,0 +1,136 @@
+# heuropt
+
+A practical Rust toolkit for implementing heuristic single-objective,
+multi-objective, and many-objective optimization algorithms.
+
+`heuropt` is **not** a research framework full of abstract machinery — it is a
+small set of concrete types, a handful of simple traits, and a few reference
+algorithms. The goal: an entry-level Rust engineer can define a problem, run a
+built-in optimizer, or implement a new optimizer without learning any
+framework concepts.
+
+## Installation
+
+```toml
+[dependencies]
+heuropt = "0.1"
+
+# Optional: derive serde::{Serialize, Deserialize} on the core data types.
+# heuropt = { version = "0.1", features = ["serde"] }
+```
+
+## Define a problem
+
+```rust
+use heuropt::prelude::*;
+
+struct SchafferN1;
+
+impl Problem for SchafferN1 {
+    type Decision = Vec<f64>;
+
+    fn objectives(&self) -> ObjectiveSpace {
+        ObjectiveSpace::new(vec![
+            Objective::minimize("f1"),
+            Objective::minimize("f2"),
+        ])
+    }
+
+    fn evaluate(&self, x: &Vec<f64>) -> Evaluation {
+        let v = x[0];
+        Evaluation::new(vec![v * v, (v - 2.0).powi(2)])
+    }
+}
+```
+
+## Run NSGA-II
+
+```rust
+use heuropt::prelude::*;
+
+# struct SchafferN1;
+# impl Problem for SchafferN1 {
+#     type Decision = Vec<f64>;
+#     fn objectives(&self) -> ObjectiveSpace {
+#         ObjectiveSpace::new(vec![Objective::minimize("f1"), Objective::minimize("f2")])
+#     }
+#     fn evaluate(&self, x: &Vec<f64>) -> Evaluation {
+#         Evaluation::new(vec![x[0] * x[0], (x[0] - 2.0).powi(2)])
+#     }
+# }
+let initializer = RealBounds::new(vec![(-5.0, 5.0)]);
+let variation = GaussianMutation { sigma: 0.2 };
+let config = Nsga2Config { population_size: 60, generations: 80, seed: 42 };
+let mut optimizer = Nsga2::new(config, initializer, variation);
+let result = optimizer.run(&SchafferN1);
+
+println!("Pareto front size: {}", result.pareto_front.len());
+```
+
+See `examples/toy_nsga2.rs` for the full version.
+
+## Implement a custom optimizer
+
+A new optimizer is just an implementation of `Optimizer<P>`:
+
+```rust
+use heuropt::prelude::*;
+
+struct MyOptimizer { /* state */ }
+
+impl<P> Optimizer<P> for MyOptimizer
+where
+    P: Problem<Decision = Vec<f64>>,
+{
+    fn run(&mut self, problem: &P) -> OptimizationResult<P::Decision> {
+        // Generate candidates.
+        // Evaluate them with `problem.evaluate(...)`.
+        // Keep the best, or maintain a Pareto archive.
+        // Return an OptimizationResult.
+        # OptimizationResult::new(
+        #     Population::new(Vec::new()),
+        #     Vec::new(),
+        #     None,
+        #     0,
+        #     0,
+        # )
+    }
+}
+```
+
+A complete worked example is in `examples/custom_optimizer.rs`.
+
+## Current algorithms
+
+- `RandomSearch` — sample-evaluate-keep baseline.
+- `Paes` — a small (1+1) Pareto Archived Evolution Strategy.
+- `Nsga2` — the canonical Pareto-based evolutionary algorithm.
+- `DifferentialEvolution` — DE/rand/1/bin for single-objective real-valued
+  problems.
+
+Plus reusable utilities: `pareto_compare`, `pareto_front`, `best_candidate`,
+`non_dominated_sort`, `crowding_distance`, `ParetoArchive`, and the metrics
+`spacing` and `hypervolume_2d`.
+
+## Design philosophy
+
+- **Concrete data, small trait surface.** `Problem`, `Optimizer`, `Initializer`,
+  `Variation` are the only traits a user interacts with day-to-day. Everything
+  else is plain structs.
+- **No type hell.** No trait objects in the core path, no GATs, no HRTBs in
+  user-facing APIs, no generic-RNG plumbing — `Rng` is a single concrete type
+  alias.
+- **Readable algorithms.** Built-ins are written for clarity, not maximum
+  abstraction reuse. `RandomSearch` is the recommended file to read before
+  writing your own optimizer.
+- **One crate first.** No premature splitting into `-core`/`-algorithms`/
+  `-operators`. Split later if the crate grows.
+- **Panic on programmer error.** Invalid configuration panics with a clear
+  message in v1; the API may grow `Result`-returning variants later if the
+  base API proves useful.
+
+See `docs/heuropt_tech_design_spec.md` for the full design rationale.
+
+## License
+
+MIT.