feat(filter_protocol): Filter Editor SDK contract + registry service

Exposes the FilterTransform contract and the FilterRegistryService as a
shared cross-DSO surface so the host (PJ4) and the Filter Editor toolbox
plugin can agree on a single set of builtin transforms — same registry,
same instances, no duplicated factories.

- pj_plugins/filter_protocol/sdk:
  - FilterTransform + builtin_transforms (12 vendored stateful operators:
    scale, offset, abs, derivative, integral, moving_average, moving_rms,
    moving_variance, outlier_removal, samples_counter, binary_filter,
    time_since_previous, none).
  - FilterTransformFactory: typed registry keyed by string id; create()
    returns shared_ptr<FilterTransform> with a deleter that pins the
    library_owner (cross-DSO lifetime safety).
  - FilterRegistryService C ABI + FilterRegistryView C++ wrapper that
    plugins use to register transforms during bind() and resolve them
    later. Live instances bridged via a static catalogue so the per-id
    deleter handed back to plugins can free them after the host shuts
    down.

- pj_plugins/host/filter_registry_host.hpp: thin adapter that wraps a
  FilterTransformFactory in the service vtable, with a noexcept-correct
  C ABI and idempotent unregister.

Author: pabloinigoblasco

pj_base/include/pj_base/builtin/image_annotations.hpp

@@ -19,6 +19,7 @@
 #include <string>
 #include <vector>
 
+#include "pj_base/point2.hpp"
 #include "pj_base/types.hpp"
 
 namespace PJ {
@@ -32,12 +33,8 @@ enum class AnnotationTopology : uint8_t {
   kLineLoop,   ///< Like LineStrip but closes back to the first point. 4-point loop = rectangle.
 };
 
-/// 2D point in image-pixel coordinates (origin top-left).
-struct Point2 {
-  double x = 0.0;
-  double y = 0.0;
-  bool operator==(const Point2&) const = default;
-};
+// `Point2` lives in pj_base/point2.hpp (generic 2D vocab type). In this header
+// it's used in image-pixel coordinates (origin top-left).
 
 /// 8-bit per-channel RGBA color. a=0 means transparent / disabled.
 struct ColorRGBA {

pj_base/include/pj_base/point2.hpp

@@ -0,0 +1,18 @@
+// Copyright 2026 Davide Faconti
+// SPDX-License-Identifier: Apache-2.0
+#pragma once
+
+// Plain (x, y) 2D point. Generic double-precision vocab type — used by image
+// annotations (pixel coordinates), Filter Editor transforms (time/value
+// samples), and any other 2D context where a tagged shape would be overkill.
+// The semantic of x / y is owned by the caller.
+
+namespace PJ::sdk {
+
+struct Point2 {
+  double x = 0.0;
+  double y = 0.0;
+  bool operator==(const Point2&) const = default;
+};
+
+}  // namespace PJ::sdk

pj_plugins/CMakeLists.txt

@@ -17,6 +17,25 @@ target_link_libraries(pj_plugin_loader_detail PUBLIC pj_base)
 
 add_subdirectory(dialog_protocol)
 
+# Filter Editor transform contract (header-only) — plugins implement the 12
+# concrete strategies and self-register with the factory at load time. See
+# pj_plugins/filter_protocol/include/pj_plugins/sdk/filter_transform.hpp.
+add_library(pj_filter_sdk INTERFACE)
+target_compile_features(pj_filter_sdk INTERFACE cxx_std_20)
+target_include_directories(pj_filter_sdk INTERFACE
+  $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/filter_protocol/include>
+  $<INSTALL_INTERFACE:include>
+)
+# Builtin strategies (in the plugin) use nlohmann/json for saveParams /
+# loadParams; propagate the dep so consumers get it transitively. Also pulls
+# pj_base for the C ABI primitives (PJ_string_view_t, PJ_error_t, PJ_NOEXCEPT)
+# referenced by the filter registry service.
+target_link_libraries(pj_filter_sdk INTERFACE pj_base nlohmann_json::nlohmann_json)
+set_target_properties(pj_filter_sdk PROPERTIES EXPORT_NAME filter_sdk)
+add_library(plotjuggler_sdk::filter_sdk ALIAS pj_filter_sdk)
+install(DIRECTORY filter_protocol/include/ DESTINATION include)
+install(TARGETS pj_filter_sdk EXPORT plotjuggler_sdkTargets ARCHIVE DESTINATION lib LIBRARY DESTINATION lib INCLUDES DESTINATION include)
+
 # ---------------------------------------------------------------------------
 # pj_plugin_sdk — umbrella INTERFACE library that exposes the full plugin-author
 # SDK surface: C++ SDK headers (MessageParserPluginBase, ObjectIngestPolicy,
@@ -29,7 +48,7 @@ target_include_directories(pj_plugin_sdk INTERFACE
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
   $<INSTALL_INTERFACE:include>
 )
-target_link_libraries(pj_plugin_sdk INTERFACE pj_base pj_dialog_sdk)
+target_link_libraries(pj_plugin_sdk INTERFACE pj_base pj_dialog_sdk pj_filter_sdk)
 set_target_properties(pj_plugin_sdk PROPERTIES EXPORT_NAME plugin_sdk)
 add_library(plotjuggler_sdk::plugin_sdk ALIAS pj_plugin_sdk)