test: integration suite with real Traefik, redis-shared limits, streaming assertions

Adds integration/ (separate module): docker compose stack with real Traefik
v3.5 + localPlugins, mock openai/anthropic/gemini/tool providers, shared
Redis, and a raw-TCP streaming timing probe. 6/7 tests pass; streaming
incrementality fails on a confirmed external yaegi/Traefik bug (see
task-15-report.md), not a plugin defect.

Fixes 4 real Yaegi-interpretation bugs surfaced by running the plugin under
real Traefik for the first time (none caught by go test or yaegi-check,
which never exercises chat/embeddings/streaming/Redis):

- .traefik.yml: add basePkg (Traefik derives the yaegi binding identifier
  from the module path, not the package's own declared name; without this
  override the plugin never loads at all).
- registry.go: resolveAgainst's multi-value return panicked yaegi on an
  inline string concatenation; assign to a local first.
- routes_unified.go: errors.As panicked yaegi on interpreted pointer error
  types; use plain type assertions (both types are always returned bare,
  never wrapped).
- sse.go: readSSE's dispatch closure hit the same multi-value-assignment
  panic as registry.go; split into separate statements.
- resp.go: respErr as a defined string type was indistinguishable from a
  plain string in a yaegi type assertion, misreading every successful
  Redis SELECT reply as failed and silently disabling rate limiting;
  changed to a struct.

routes_unified.go

@@ -225,14 +225,21 @@ func (g *Gateway) handleAdapterError(sw *statusTrackingWriter, err error, provid
 		return
 	}
 
-	var perr *providerHTTPError
-	if errors.As(err, &perr) {
+	// Plain type assertions, not errors.As: providerHTTPError and
+	// translateError are both always returned bare from every adapter and
+	// translate_*.go call site — never wrapped via fmt.Errorf("%w", ...) —
+	// so errors.As's unwrap-chain walk buys nothing here, and yaegi
+	// v0.16.1 panics ("errors: *target must be interface or implement
+	// error") calling errors.As with an interpreted pointer type as its
+	// target, even though that type's Error() method is right there.
+	// Verified empirically under real Traefik (Task 15's integration
+	// suite); tools/yaegi-check never exercises this call path.
+	if perr, ok := err.(*providerHTTPError); ok {
 		writeProviderUpstreamError(sw, providerName, perr)
 		return
 	}
 
-	var terr *translateError
-	if errors.As(err, &terr) {
+	if terr, ok := err.(*translateError); ok {
 		status := http.StatusBadRequest
 		if terr.notSupported {
 			status = http.StatusNotImplemented