feat(core): expose read-only config view, generic getter and redacted dump

补齐 Value/String/Bool/Int/Duration/Strings/WasSet/Origin 只读访问器与
按 secret 脱敏的 Entries 导出，新增 core.ConfigGet[T] 泛型读取入口，并用
编译期断言钉住 ConfigRegistry 对 ConfigExtension 的完整实现。

backend/core/config.go

@@ -0,0 +1,31 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package core
+
+import (
+	"fmt"
+
+	"Wavelet/core/extpoints"
+)
+
+// ConfigGet reads one resolved configuration value with its declared type. It is the
+// generic counterpart of the fallback accessors on ConfigView, used when a caller must
+// distinguish "unset" from "set to the zero value".
+func ConfigGet[T any](view extpoints.ConfigView, key string) (T, error) {
+	var zero T
+	if view == nil {
+		return zero, extpoints.ErrConfigNotResolved
+	}
+
+	raw, ok := view.Value(key)
+	if !ok {
+		return zero, fmt.Errorf("%w: %s", extpoints.ErrConfigUnknownKey, key)
+	}
+
+	value, ok := raw.(T)
+	if !ok {
+		return zero, fmt.Errorf("%w: key %q holds %T, want %T", extpoints.ErrConfigType, key, raw, zero)
+	}
+	return value, nil
+}