Seventh TY7 sliver. The Assert::guard recognizer now distributes an
outstanding `not` through `and`/`or` (De Morgan): not (p and q) →
(not p) or (not q); not (p or q) → (not p) and (not q) — the case that
narrows, e.g. not (x is Cat or x is Dog) over Cat or Dog or Bird ⇒ Bird.

This rested on making conjunction compose by intersection
(merge_conjunction / intersect, the dual of residual): a local
constrained by both sides of an `and` narrows to the common leaves, and
disjoint constraints intersect to Nothing (unsatisfiable guard → dead
continuation, E0720). Fixes a latent imprecision in the plain `p and q`
row (last-write-wins concatenation gave the wrong type for a
doubly-constrained local). The value-algebra ==/!= rows stay un-flipped
(the two algebras never mix in one expression).

Golden: assert_guard_de_morgan_narrows.cktest (trapped, ratchet holds at
112); merge_conjunction / intersect unit tests in narrow.rs.

@@ -473,0 +525,4 @@
+            } => {
+                cx.results.locals.insert(*local, scrutinee_ty.clone());
+            }
+            _ => {}