feat(citygen): City::validate and seed sweep tests

Also guarantees one building per district and repairs interiors split
by crossing partitions, both found by the new validation.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

crates/citygen/tests/seeds.rs

@@ -0,0 +1,64 @@
+//! Determinism and invariant sweeps over many seeds.
+
+use citygen::*;
+
+fn sweep(preset: SizePreset, seeds: std::ops::Range<u64>) {
+    for seed in seeds {
+        let p = GenParams {
+            seed,
+            size: preset,
+            water: match seed % 3 {
+                0 => WaterMode::Bay,
+                1 => WaterMode::River,
+                _ => WaterMode::None,
+            },
+            districts: 4 + (seed % 9) as u8,
+            ..Default::default()
+        };
+        if p.validate().is_err() {
+            continue;
+        }
+        let a = Generator::run(p.clone()).unwrap_or_else(|e| panic!("seed {seed}: {e}"));
+        let b = Generator::run(p).unwrap();
+        assert_eq!(
+            postcard::to_allocvec(&a).unwrap(),
+            postcard::to_allocvec(&b).unwrap(),
+            "seed {seed} not deterministic"
+        );
+        a.validate()
+            .unwrap_or_else(|v| panic!("seed {seed}: {v:?}"));
+    }
+}
+
+#[test]
+fn small_seeds() {
+    sweep(SizePreset::Small, 0..50);
+}
+
+#[test]
+fn medium_seeds() {
+    sweep(SizePreset::Medium, 1000..1010);
+}
+
+#[test]
+fn large_seeds() {
+    sweep(SizePreset::Large, 2000..2003);
+}
+
+#[test]
+#[ignore]
+fn extended_seeds() {
+    sweep(SizePreset::Small, 100..250);
+    sweep(SizePreset::Medium, 3000..3050);
+}
+
+#[test]
+fn medium_generates_under_two_seconds() {
+    let t = std::time::Instant::now();
+    Generator::run(GenParams {
+        seed: 77,
+        ..Default::default()
+    })
+    .unwrap();
+    assert!(t.elapsed().as_secs_f32() < 2.0, "took {:?}", t.elapsed());
+}