✅ Assert README example results

Assign every documented matcher result to a variable and assert it with
`assert.equal`, so the compiled README doc-tests verify behavior instead
of merely running the code. The examples import `node:assert` themselves
to stay copy-pasteable; the generator now merges that import into its
prelude assert rather than rejecting it (only `node:test` remains
reserved).

README.md

@@ -47,6 +47,7 @@ builder. Calling the builder with a handler map keyed by `T`'s members returns a
 matcher: a function from `T` to the common return type.
 
 ```ts
+import { strict as assert } from "node:assert";
 import { getPrimitiveUnionMatcher } from "tiny-pattern-ts";
 
 const matchAnswer = getPrimitiveUnionMatcher<"yes" | "no">();
@@ -56,13 +57,15 @@ const reply = matchAnswer({
     no: () => "declined",
 });
 
-reply("yes"); // "agreed"
+const answer = reply("yes");
+assert.equal(answer, "agreed");
 ```
 
 Add a fallback as the second argument to leave members unhandled; the fallback
 receives the remainder:
 
 ```ts
+import { strict as assert } from "node:assert";
 import { getPrimitiveUnionMatcher } from "tiny-pattern-ts";
 
 const matchLabel = getPrimitiveUnionMatcher<"yes" | "no" | "maybe">();
@@ -71,6 +74,12 @@ const label = matchLabel(
     { yes: () => "agreed", no: () => "declined" },
     (other) => `not sure: ${other}`, // other: "maybe"
 );
+
+const answer = label("yes");
+assert.equal(answer, "agreed");
+
+const fallback = label("maybe");
+assert.equal(fallback, "not sure: maybe");
 ```
 
 `getPrimitiveUnionMatcherW` is the same builder, but the matcher's return type
@@ -83,6 +92,7 @@ function takes the discriminant property's name and returns the handler-map
 builder, keyed by that property's tags.
 
 ```ts
+import { strict as assert } from "node:assert";
 import { getTaggedUnionMatcher } from "tiny-pattern-ts";
 
 type Shape =
@@ -95,7 +105,11 @@ const area = matchShape({
     square: (s) => s.side ** 2,
 });
 
-area({ kind: "circle", radius: 2 });
+const circleArea = area({ kind: "circle", radius: 2 });
+assert.equal(circleArea, Math.PI * 4);
+
+const squareArea = area({ kind: "square", side: 3 });
+assert.equal(squareArea, 9);
 ```
 
 `getTaggedUnionMatcherW` is the widening counterpart, exactly as in the