feat: package as Conan recipe with plotjuggler_core:: CMake components

[facontidavide]

Summary

• Renames the installable CMake package from PlotJugglerSDK to plotjuggler_core, with audience-aligned components: base, datastore, plugin_sdk (plugin authors), plugin_host (host loaders).
• Adds conanfile.py so downstream projects (e.g. pj-official-plugins) can resolve via find_package(plotjuggler_core REQUIRED COMPONENTS plugin_sdk) + requires plotjuggler_core/1.0.0 instead of pulling source through CPM.
• Ships PjPluginManifest.cmake alongside the package via cmake_build_modules, so pj_emit_plugin_manifest() is available to consumers without copying the helper.

Why

pj-official-plugins currently pulls plotjuggler_core source via CPMAddPackage and builds it in-tree on every consumer build. That works, but it forecloses on binary caching, version pinning via Conan, and consumption from non-CPM build systems. The existing PJ_INSTALL_SDK=ON path only installed pj_base (mis-named DataSource) + pj_dialog_sdk (Dialog) — too narrow to support a plugin author, and the names collide with the DataSource plugin family.

This PR closes both gaps.

Components and audiences

Component	Audience	Pulls in
plotjuggler_core::base	always available	pj_base
plotjuggler_core::datastore	optional (with_datastore)	pj_datastore + fmt + tsl-robin-map + nanoarrow
plotjuggler_core::plugin_sdk	plugin authors (default)	pj_base + pj_dialog_sdk + pj_plugin_sdk headers + nlohmann_json + auto-loaded pj_emit_plugin_manifest()
plotjuggler_core::plugin_host	host app + test rigs (with_host)	every pj_*_host loader + catalogs + pj_dialog_library

pj_dialog_protocol is exported as an internal target but transitively reachable through plugin_sdk and plugin_host — not advertised as a public component.

Consumer example (post-merge usage)

find_package(plotjuggler_core REQUIRED COMPONENTS plugin_sdk)
add_library(my_plugin SHARED my_plugin.cpp)
target_link_libraries(my_plugin PRIVATE plotjuggler_core::plugin_sdk)
pj_emit_plugin_manifest(my_plugin FAMILY data_source MANIFEST_FILE manifest.json)

# conanfile.py
requires = ("plotjuggler_core/1.0.0",)

Notable mechanical changes

• All exported targets now use BUILD_INTERFACE/INSTALL_INTERFACE include directories (required for the EXPORT to survive install).
• Created a new pj_plugin_host INTERFACE umbrella that aggregates all host loaders.
• Extended pj_plugin_sdk INTERFACE umbrella to also bundle pj_dialog_sdk so plugin authors link one target.
• Installed pj_plugins/include/, pj_datastore/include/, and pj_plugins/dialog_protocol/include/ trees; previously dialog headers were installed via explicit install(FILES ...).
• cmake/PjPluginManifest.cmake now include(GNUInstallDirs) itself so consumers don't have to.
• conanfile.txt removed (replaced by conanfile.py which handles both packaging and local dev via with_tests / with_parquet_example options).

Test plan

• ./build.sh — full in-tree RelWithDebInfo build passes
• ./test.sh — 48/48 tests pass
• ./test_sdk_install.sh — installs, all four components resolve via find_package, examples/sdk_consumer builds + emits manifest sidecar
• conan create . — recipe builds and packages cleanly
• Out-of-tree Conan consumer (conanfile.txt: requires plotjuggler_core/1.0.0) resolves and finds all components
• pj-official-plugins migration from CPM to find_package(plotjuggler_core) + requires plotjuggler_core/1.0.0 (follow-up PR)
• Docs update (README, USER_GUIDE) for the new package layout (follow-up)
• Publish to a Conan remote (cloudsmith?) once a tagged release lands

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