feat(benchmarks): add correctness assertion (#31)

* feat(benchmarks): add correctness assertion - proves less code is not broken code

The existing benchmark measures lines-of-code (loc.js) but never checks
whether the generated code actually works. This adds a functional
correctness gate (correctness.js) that extracts code from fenced blocks
and runs per-task checks:

- email validator: spawns Python, asserts accept/reject on 5 inputs
- debounce: spawns Node, asserts delayed execution + reset on re-call
- csv sum: spawns Python with a test CSV, asserts correct total (351)
- countdown (React): structural check (useState + useEffect + decrement)
- rate limiter (FastAPI): structural check (limit logic + framework usage)

12 unit tests (node:test) cover good/bad outputs for every task plus the
unknown-task edge case. Existing tests and rule-copy checks unaffected.

* fix: address review feedback

- csv check: use regex lookaround instead of substring match to prevent
  false positives (e.g. 13510 containing '351')
- ratelimit: fix operator precedence in block finder by adding parens
  around the || inside the !b.lang guard
- README: note that React/FastAPI checks are structural only, add
  prerequisites section (Python 3, pandas, Node.js 18+)
- test: add regression test for csv substring false positive

benchmarks/promptfooconfig.yaml

@@ -29,6 +29,9 @@ defaultTest:
     - type: javascript
       value: file://loc.js
       metric: code_loc
+    - type: javascript
+      value: file://correctness.js
+      metric: correct
 
 tests:
   - vars: { task: "Write me a Python function that validates email addresses." }