End-to-end: harness Playwright suite green (14 pass, 1 skip)

After the React-codegen rework, ran the full e2e harness against the
docker-stack backend. Surfaced and fixed real friction:

mizan-base/src/index.ts (kernel):
- MizanError now parses both error envelopes — the FastAPI shape
  ({"error": {"code", "message", "details"}}) and the Django shape
  ({"error": true, "code", "message", "details"}). Exposes .code and
  .details on the thrown error so consumer code can branch on them.
  This was needed for the harness's `instanceof MizanError && error.code
  === 'NOT_FOUND'` pattern to work; the previous MizanError only carried
  status + raw body, leaving callers to parse the body themselves.

examples/django-react-site/Dockerfile.test:
- Backend image now copies and installs cores/mizan-python before
  installing mizan-django (which imports from mizan_core after the
  Layer 1 extraction).

harness/src/fixtures.tsx:
- useRun helper updated for the new mutation-hook shape: pulls
  { mutate } off the hook result instead of treating the hook return
  as a callable. Same for ValidationError fixture.

mizan.spec.ts:
- DjangoError → MizanError (kernel error class is backend-agnostic).
- Form tests removed (forms codegen deferred per Blazr scope).
- Channel test marked test.skip (channels deferred per Blazr scope).

.gitignore: ignore Playwright test-results/.

Final verification across all surfaces:
- mizan-core unit:       15/15
- mizan-django unit:     348 pass, 21 skip
- mizan-fastapi unit:    11/11
- mizan-ts edge-compat:  34/34 (cross-language HMAC pin)
- harness e2e (Playwright): 14/15 (1 skip = channels deferred)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

examples/django-react-site/Dockerfile.test

@@ -7,6 +7,10 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     gcc \
     && rm -rf /var/lib/apt/lists/*
 
+# Install mizan-core (shared Python protocol primitives) first
+COPY cores/mizan-python/ /app/mizan-core/
+RUN pip install --no-cache-dir /app/mizan-core
+
 # Install mizan from local source with channels support
 COPY backends/mizan-django/ /app/django/
 RUN pip install --no-cache-dir /app/django[channels] daphne