test: cover non-finite results and color-group disjointness

The two gaps #26 named that were never filled.

NonFiniteResult had no test at all — the name appeared in `tests/` only
inside a doc comment, and it is the sub-claim in that issue's title. It
turns out to be very much reachable, and from *finite* inputs: sigma at
1e300, beta at 1e300, sigma at 1e-300, score_sigma at 1e-300, and scores
at 1e308 all overflow inside inference, where the boundary checks cannot
see them. That matters because the failure is silent by default — NaN
fails every comparison, so a naive `step < epsilon` reads a NaN step as
converged, which is why the crate has `step_converged`/`step_is_finite`.
Pinned from outside, including that `converge_partial` does not launder a
breakdown into an `Ok`, and with a control asserting merely extreme
parameters still converge so the suite cannot pass by always failing.

Color-group disjointness was #26's fourth acceptance criterion and had
only five hand-written cases. Now a proptest over three shapes: a dense
pool where collisions force colors to multiply, a sparse one where most
events are independent, and repeated members within a single event.

Two of my first assertions were wrong about the code rather than the
reverse. A competitor named twice *within* one event is not a collision —
`color_greedy` collects each event's members into a set for that reason.
And contiguity is not a property of `color_greedy`: it holds only after
`recompute_color_groups` reorders events so each color occupies one
range. The test now asserts what is actually promised — that the reorder
is always *possible*, since the parallel sweep slices `&mut` sub-ranges
from those groups and overlapping ranges would be unsound.

Refs #26

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011hcFjNDmHXZF8URGLku5zZ

src/color_group.rs

@@ -191,3 +191,121 @@ mod tests {
         assert_eq!(cg.total_events(), 4);
     }
 }
+
+#[cfg(test)]
+mod properties {
+    use std::collections::HashSet;
+
+    use proptest::prelude::*;
+
+    use super::*;
+
+    /// The property the whole parallel sweep rests on: two events sharing a
+    /// competitor must never land in the same color, because a color group is
+    /// run concurrently and two events touching one competitor would race.
+    ///
+    /// Hand-written cases cover the shapes someone thought of. This covers the
+    /// ones nobody did — the correctness of `sweep_color_groups` depends on it
+    /// holding for every input, not for five.
+    fn check(events: &[Vec<usize>]) {
+        let groups = color_greedy(events.len(), |ev| {
+            events[ev]
+                .iter()
+                .copied()
+                .map(Index::from)
+                .collect::<Vec<_>>()
+        });
+
+        // Disjointness *between events* within a color. Deduplicated per
+        // event, because one event legitimately naming a competitor twice is
+        // not a collision — `color_greedy` collects each event's members into
+        // a set for exactly that reason.
+        for color in 0..groups.n_colors() {
+            let mut seen: HashSet<usize> = HashSet::new();
+            for &ev in &groups.groups[color] {
+                let members: HashSet<usize> = events[ev].iter().copied().collect();
+                for competitor in members {
+                    assert!(
+                        seen.insert(competitor),
+                        "competitor {competitor} shared by two events in color {color}"
+                    );
+                }
+            }
+        }
+
+        // Every event is assigned exactly once. Without this, a partition that
+        // dropped events would satisfy disjointness trivially.
+        let mut assigned: Vec<usize> = groups.groups.iter().flatten().copied().collect();
+        assigned.sort_unstable();
+        assert_eq!(assigned, (0..events.len()).collect::<Vec<_>>());
+        assert_eq!(groups.total_events(), events.len());
+
+        // No empty colors: one would waste a sweep and make `n_colors`
+        // misleading.
+        for (color, group) in groups.groups.iter().enumerate() {
+            assert!(!group.is_empty(), "color {color} is empty");
+        }
+
+        // Contiguity is not a property of `color_greedy` — it holds only after
+        // `recompute_color_groups` reorders the events so each color occupies
+        // one range. What must always hold is that the reorder is *possible*:
+        // relabelling events in group order yields contiguous groups. The
+        // parallel sweep slices `&mut` sub-ranges from those, so if this ever
+        // failed the reorder would produce overlapping ranges.
+        let mut next = 0usize;
+        let relabelled: Vec<Vec<usize>> = groups
+            .groups
+            .iter()
+            .map(|group| {
+                group
+                    .iter()
+                    .map(|_| {
+                        let i = next;
+                        next += 1;
+                        i
+                    })
+                    .collect()
+            })
+            .collect();
+        assert!(ColorGroups { groups: relabelled }.groups_are_contiguous());
+    }
+
+    proptest! {
+        #![proptest_config(ProptestConfig::with_cases(512))]
+
+        /// Small competitor pool, so collisions are common and colors are
+        /// forced to multiply.
+        #[test]
+        fn colors_are_disjoint_on_a_dense_pool(
+            events in prop::collection::vec(
+                prop::collection::vec(0usize..6, 1..4),
+                0..20,
+            )
+        ) {
+            check(&events);
+        }
+
+        /// Wide pool, so most events are independent and land in one color.
+        #[test]
+        fn colors_are_disjoint_on_a_sparse_pool(
+            events in prop::collection::vec(
+                prop::collection::vec(0usize..200, 1..6),
+                0..30,
+            )
+        ) {
+            check(&events);
+        }
+
+        /// Repeated competitors within one event must not confuse the
+        /// member-set bookkeeping.
+        #[test]
+        fn colors_are_disjoint_with_repeated_members(
+            events in prop::collection::vec(
+                prop::collection::vec(0usize..3, 1..8),
+                0..15,
+            )
+        ) {
+            check(&events);
+        }
+    }
+}