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.

.gitignore

@@ -85,3 +85,7 @@ examples/folio-web/.vite/
 
 /talk/
 keys/*
+
+# Local experiments against a subject application, its own Go module so the
+# product module never compiles it.
+/playground/