👷 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/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