Add the Cut 6d mutation suite

Five entries, each restoring the permissiveness the shape it pins
removed: M23 and M24 the one-per-subject and one-per-repo keys, M25 and
M26 the sequence anywhere but last, M27 a nested subject stripped of its
own sequence on the way into the key. Each dies on the test named beside
it, M0 green.

The file records what has no entry and why: the `n` marker is a fixture
pin, the `u32` type has no runtime mutation, and date-invariance is
covered by M24's test asserting a stewardship local is exactly two parts.

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

tools/eureka-cut6d-mutations.psd1

@@ -0,0 +1,62 @@
+# Eureka Cut 6d mutation suite: resolution history and stewardship by
+# sequence. Entries only; the harness is tools/eureka-mutations.ps1.
+#
+#   powershell -File tools/eureka-mutations.ps1 `
+#       -Entries tools/eureka-cut6d-mutations.psd1 `
+#       -Target epiphany-pipeline/src/lib.rs `
+#       -Test 'cargo test -p epiphany-pipeline --lib'
+#
+# Each entry restores the permissiveness the shape it pins removed: M23 and
+# M24 restore the one-per-subject and one-per-repo keys this cut retired, M25
+# and M26 put the sequence anywhere but last, and M27 drops a nested subject's
+# own sequence on the way into the key. Cut 6b's M2 already covers the
+# subject's kind on the same line and is re-anchored there, not repeated here.
+#
+# Stated limits, so a later reader does not mistake silence for coverage. The
+# `n` marker itself has no entry: it is pinned only by the sample-key
+# equalities in `keys_are_derived_and_mismatch_refuses` and the two prefix
+# literals, a fixture pin like 6c's M21, and changing a marker restores no
+# permissiveness. The `u32` type has no runtime mutation. Date-invariance has
+# none of its own either: putting `assigned_on` into the stewardship key is
+# not a permissiveness the old code had, and M24's test asserts the local's
+# part count is exactly two, so a mutant that appends the date as a third part
+# dies there.
+@{
+    Mutations = @(
+        @{
+            Id   = 'M23'
+            Rule = 'A subject''s resolutions are distinct records: the sequence is in the key (Q17 B).'
+            Test = 'tests::a_subject_keeps_every_resolution_it_had'
+            Old  = '            let parts = std::iter::once(value.subject.kind.name()).chain(subject_local.split(''.'')).chain(std::iter::once(sequence.as_str())).collect::<Vec<_>>();'
+            New  = '            let parts = std::iter::once(value.subject.kind.name()).chain(subject_local.split(''.'')).collect::<Vec<_>>();'
+        },
+        @{
+            Id   = 'M24'
+            Rule = 'A repo''s stewardships on a mind are distinct records: the sequence is in the key (Q18 A, Q20 A).'
+            Test = 'tests::a_repo_keeps_every_stewardship_it_had'
+            Old  = 'local(&key_field, &[&key_segment(&value.repo.0), &sequence])?)'
+            New  = 'local(&key_field, &[&key_segment(&value.repo.0)])?)'
+        },
+        @{
+            Id   = 'M25'
+            Rule = 'A stewardship''s sequence is the last part, so a repo''s assignments share a prefix (D2).'
+            Test = 'tests::a_repo_keeps_every_stewardship_it_had'
+            Old  = 'local(&key_field, &[&key_segment(&value.repo.0), &sequence])?)'
+            New  = 'local(&key_field, &[&sequence, &key_segment(&value.repo.0)])?)'
+        },
+        @{
+            Id   = 'M26'
+            Rule = 'A resolution''s sequence is the last part, so a subject''s resolutions share a prefix (D2).'
+            Test = 'tests::a_subjects_resolutions_share_a_prefix_no_other_key_has'
+            Old  = '            let parts = std::iter::once(value.subject.kind.name()).chain(subject_local.split(''.'')).chain(std::iter::once(sequence.as_str())).collect::<Vec<_>>();'
+            New  = '            let parts = std::iter::once(value.subject.kind.name()).chain(std::iter::once(sequence.as_str())).chain(subject_local.split(''.'')).collect::<Vec<_>>();'
+        },
+        @{
+            Id   = 'M27'
+            Rule = 'A nested subject is recovered whole: a resolution of a resolution keys the inner key, sequence included (defect 3, under the new shape).'
+            Test = 'tests::a_resolution_of_a_resolution_reads_back'
+            Old  = '            let parts = std::iter::once(value.subject.kind.name()).chain(subject_local.split(''.'')).chain(std::iter::once(sequence.as_str())).collect::<Vec<_>>();'
+            New  = '            let parts = std::iter::once(value.subject.kind.name()).chain(subject_local.rsplit_once(''.'').map_or(subject_local, |(head, _)| head).split(''.'')).chain(std::iter::once(sequence.as_str())).collect::<Vec<_>>();'
+        }
+    )
+}