👷 Gate CI at 100% coverage

test:ci now runs c8 with --all --include "src/**/*.ts" --100, so the
build job fails when any runtime file under src/ is untested. --all is
what makes the gate non-vacuous: without it c8 counts only the files the
suite happened to load, and a new untested module stays invisible.

Add src/index.test.ts to load the public barrel, which was previously
never imported at runtime and so read as 0% under --all. matcher-shared.ts
is types-only (an empty runtime image) and carries a file-level c8 ignore
with the reason.

Why 100% and the rejected alternatives: development/ci.md § Coverage
threshold.

src/index.test.ts

@@ -0,0 +1,27 @@
+import { strict as assert } from "node:assert";
+import { test } from "node:test";
+
+import { expectTypeOf } from "expect-type";
+
+import {
+    getPrimitiveUnionMatcher,
+    getPrimitiveUnionMatcherW,
+    getTaggedUnionMatcher,
+    getTaggedUnionMatcherW,
+} from "./index.ts";
+
+// The published entry point is the barrel (`package.json` exports
+// `./dist/index.js`), so every factory must be reachable from here. Importing it
+// also loads the module, which is what lets c8's `--all` measure it — see
+// development/ci.md § Coverage threshold.
+test("index: the public entry point re-exports every matcher factory", () => {
+    // Assert
+    expectTypeOf(getPrimitiveUnionMatcher).toBeFunction();
+    assert.equal(typeof getPrimitiveUnionMatcher, "function");
+    expectTypeOf(getPrimitiveUnionMatcherW).toBeFunction();
+    assert.equal(typeof getPrimitiveUnionMatcherW, "function");
+    expectTypeOf(getTaggedUnionMatcher).toBeFunction();
+    assert.equal(typeof getTaggedUnionMatcher, "function");
+    expectTypeOf(getTaggedUnionMatcherW).toBeFunction();
+    assert.equal(typeof getTaggedUnionMatcherW, "function");
+});

src/matcher-shared.ts

@@ -1,3 +1,4 @@
+/* c8 ignore start -- types only: the module has no runtime image to cover */
 import type { IsLiteral, IsNever, ValueOf } from "type-fest";
 
 // The primitive-union and tagged-union matchers differ in their universe, but the