Freeze the Go implementation as a test-only oracle, with vectors

oracle/go is the Go code at 4c1507c, moved unchanged, plus test-only
generators that record what it computes. tests/vectors holds the
deterministic crypto/store vectors and synthetic fixtures they produce.

Co-Authored-By: Claude <noreply@anthropic.com>

tests/vectors/key-with-decoy.pem

@@ -0,0 +1,7 @@
+-----BEGIN VAULT KEY-----
+VkFVTFRLRVkxAAAAAwABAAAEgOQrPuXO8L/uhqn/gPncOQDW1UGohTv1BUEG37eX
+g/wZRVl2Nptu6xbqAhL1xgLstttod4sT5iNfFxvYsZnSQLsCMjaNwO5/a8rn8fOw
+QhRKx38573ADf3kA8Hv9qiykMgEAa3Ml9Qz6vlqB5gRWuK4meXSpEuxdz7XcpxIn
+lOo0jYKzagOhNM40jTBkiQmJvvYJIWt48Ckl/goDk+GQRY1SWFRW0ioQF89dHK7D
+XlBfBz9o
+-----END VAULT KEY-----