chore: move subject-specific experiments to an ignored playground module

test/okcredit and scripts/corpus were untracked and unignored, so a single
git add -A would commit them. They move to playground/, which declares its own
go.mod so `go test ./...` at the root stops compiling a 566-line package it
only skips.

Also renames the subject's bundle identifiers, used as fixture strings in two
tests of generic logic, to a neutral one.

internal/trace/writer_test.go

@@ -26,7 +26,7 @@ func TestWriteMeta_RoundTrip(t *testing.T) {
 		SpecPath:          "spec.ts",
 		BundleSHA256:      "deadbeef",
 		Platform:          "android",
-		BundleID:          "in.okcredit.merchant",
+		BundleID:          "com.fixture.merchant",
 		StartedAt:         time.Date(2026, 4, 17, 22, 30, 0, 0, time.UTC),
 		SanderlingVersion: "0.0.1",
 	}