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>

oracle/gen-vectors.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+# Regenerate the Go-oracle vectors and fixtures in tests/vectors.
+#
+# Everything is computed by the frozen Go implementation in oracle/go (see
+# oracle/README.md); nothing comes from the Rust code under test. Key files
+# and ciphertexts use fresh randomness, so regenerating changes them; the
+# deterministic vectors (*.json fields derived from fixed seeds) do not.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+out=$PWD/tests/vectors
+mkdir -p "$out"
+(cd oracle/go && VAULT_ORACLE_OUT=$out go test -count=1 -run '^TestOracleVectors$' ./ ./internal/pqcrypt ./internal/stream)
+echo "vectors written to $out"