perf: default pool admission to featureOwnership (culling), not everyDiscovery

The library default for `MutationScheduler.weightedPool(admission:)` was
`everyDiscovery` — every strategy-accepted input joins the pool and nothing
ever leaves. On stlc that floods the pool with ~2400 entries whose median
wire size is 421 chars (max 1469); mutating one of those giants rarely lands
on the relevant node.

Flip the default to `featureOwnership` (libFuzzer-style REDUCE): each feature
is owned by the smallest witness, larger owners are evicted. Measured on the
clean stlc baseline this collapses the live pool to ~20 entries of median
size 49, and the mean *executed* term shrinks 5x (323 -> 65 chars).

On the hard de Bruijn mutant shift_var_leq the flip finds the bug 20/20 at
median 4.0s vs everyDiscovery's 17/20 at 6.7s — better detection AND speed,
the direct payoff of smaller, better-targeted mutation parents.

Callers that want the old keep-everything behavior still pass
`admission: .everyDiscovery` explicitly.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: twof

Sources/PropertyTestingKit/Fuzzing/Scheduler/MutationScheduler.swift

@@ -49,7 +49,7 @@ public struct MutationScheduler: Sendable {
     ///     inputs from how many of them may stay — without it, a fine
     ///     vocabulary silently raises the population ceiling.
     public static func weightedPool(
-        admission: PoolAdmission = .everyDiscovery,
+        admission: PoolAdmission = .featureOwnership,
         policies: @escaping @Sendable () -> [any PoolPlugin] = { [] },
         burstLength: Int = 16,
         focusOnInsert: Bool = true,

Tests/PropertyTestingKitTests/Fuzzing/WeightedPoolCoreTests.swift

@@ -95,6 +95,23 @@ struct WeightedPoolCoreTests {
         #expect(core.next() == .mutate(id: 0))
     }
 
+    @Test("The default weightedPool admission culls: a same-feature, non-smaller redundant input is rejected")
+    func defaultAdmissionCulls() {
+        // The library default is feature ownership (REDUCE), not everyDiscovery:
+        // an unbounded pool of every accepted input bloats with large entries
+        // whose features a smaller input already owns. Build the core straight
+        // from the public default so this pins the default itself.
+        let core = MutationScheduler.weightedPool().makeCore()
+        // First input owns edges {1,2} (size 2) — admitted as id 0.
+        #expect(core.observe(PoolIterationOutcome(
+            source: .generated, newCoverage: SparseCoverage(indices: [1, 2]))) == 0)
+        // Second input: SAME features, SAME size — owns nothing new, steals
+        // nothing (ties don't steal), so it is rejected (nil). Under the old
+        // everyDiscovery default it would have been admitted as id 1.
+        #expect(core.observe(PoolIterationOutcome(
+            source: .generated, newCoverage: SparseCoverage(indices: [1, 2]))) == nil)
+    }
+
     @Test("Admitted entries get sequential stable IDs")
     func sequentialIDs() {
         let core = makeCore()