feat(vst): stand up VST3 instrument spike (S1) — skeleton, IPlugView↔LICE editor, REAPER bridge read

Vendor Steinberg VST3 SDK (v3.7.9_build_61); add reasampler_vst.vst3 as a second, additive build artifact with pure editor-geometry + bridge-marshal helpers under CTest.

src/vst/reaper_bridge.h

@@ -0,0 +1,63 @@
+// reaper_bridge.h — the REAPER VST-host bridge (Phase S1 read spike). THIN shell:
+// resolves REAPER API functions by name over the host context and reads the live
+// "reasampler" project ext-state. The fiddly decode lives in bridge_marshal (pure).
+//
+// VERIFIED BRIDGE MECHANISM (corrects §1a's estimate). §1a described the VST2-style
+// hostcb opcode pattern (hostcb(&effect, 0xdeadbeef, 0xdeadf00d, ...)). That is the
+// VST2 path (video_processor.h documents it for a VST2 aEffect). For a VST3 plugin the
+// bridge is exposed differently and more cleanly: REAPER passes an IHostApplication as
+// the `context` to IComponent::initialize(FUnknown* context); querying it for
+// IReaperHostApplication (vendor/reaper-sdk/sdk/reaper_vst3_interfaces.h) yields:
+//   * getReaperApi(funcname) -> resolve a REAPER API function pointer by name
+//     (the VST3 equivalent of opcode 0xdeadf00d), and
+//   * getReaperParent(3)     -> the host ReaProject* (the VST3 equivalent of the
+//     0xdeadf00e host-context fetch; 1=track, 2=take, 3=project, 4=fxdsp, 5=trackchan).
+// So a VST3 uses IReaperHostApplication, not the raw hostcb opcodes. Verified against
+// reaper_vst3_interfaces.h + reaper_plugin_functions.h at the spike.
+
+#pragma once
+
+#include <optional>
+#include <string>
+
+#include "pluginterfaces/base/funknown.h"
+
+namespace reasampler::vst {
+
+// Wraps the REAPER host bridge for a single plugin instance. Constructed cheaply;
+// connect() must be called with the initialize() context before any read. All reads
+// degrade to nullopt (never crash) when the host is not REAPER or a symbol is absent —
+// the instrument must load in non-REAPER hosts too, just without live state.
+class ReaperBridge {
+public:
+    ReaperBridge() = default;
+
+    // Bind to the host. `context` is the FUnknown* REAPER hands IComponent::initialize.
+    // Returns true when the REAPER bridge is available (host is REAPER and the ext-state
+    // API resolved). Safe to call with a null or non-REAPER context — returns false.
+    bool connect(Steinberg::FUnknown* context);
+
+    // True once connect() found the REAPER host application AND resolved the ext-state
+    // functions.
+    bool isConnected() const { return getProjExtState_ != nullptr; }
+
+    // Read a "reasampler" ext-state value by key from the host's active project.
+    // Returns nullopt when unconnected, when the project can't be resolved, or when the
+    // key is absent. This is the S1 read-spike entry point.
+    std::optional<std::string> readReasamplerExtState(const std::string& key);
+
+private:
+    // Resolved REAPER API function pointers (by name via getReaperApi). Signatures
+    // verified against reaper_plugin_functions.h.
+    using GetProjExtStateFn = int (*)(void* proj, const char* extname, const char* key,
+                                      char* valOutNeedBig, int valOutNeedBig_sz);
+    using EnumProjExtStateFn = bool (*)(void* proj, const char* extname, int idx,
+                                        char* keyOut, int keyOut_sz, char* valOut,
+                                        int valOut_sz);
+
+    void* hostApp_ = nullptr;  // IReaperHostApplication* (opaque here; used in .cpp)
+    GetProjExtStateFn getProjExtState_ = nullptr;
+    EnumProjExtStateFn enumProjExtState_ = nullptr;
+};
+
+}  // namespace reasampler::vst