👷 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.

backlog.tasks

@@ -12,9 +12,9 @@ v1.0:
     ☐ Finalize public exports in `src/index.ts`
     ☐ Document all exported types and functions
     ☐ Add JSDoc for public APIs
-☐ Test coverage meets threshold
-    ☐ Achieve 100% branch coverage on `src/primitive-union.ts`
-    ☐ Achieve 100% branch coverage on `src/index.ts`
+✔ Test coverage meets threshold @done
+    ✔ Achieve 100% branch coverage on `src/primitive-union.ts` @done
+    ✔ Achieve 100% branch coverage on `src/index.ts` @done
 
 Matcher:
 ✔ when using a union type as a property, the current behavior of tagged union matcher is @done