Add oracle harness: verb goldens and function-level vectors

Records stdout/stderr/exit/files for every impeccable verb over a fixed
corpus and replays them against an alternate implementation. Adds a loader
hook that captures per-function call vectors from the pure engine modules.

Prepared with AI assistance (Claude Code).

tests/oracle/golden/detect-fixture-json-script-error-html.json

@@ -0,0 +1,7 @@
+{
+  "stdout": "[]\n",
+  "stderr": "",
+  "exit": 0,
+  "signal": null,
+  "files": {}
+}