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/bridge_marshal.cpp

@@ -0,0 +1,78 @@
+// bridge_marshal.cpp — see bridge_marshal.h. Pure; no host types.
+
+#include "bridge_marshal.h"
+
+namespace reasampler::vst {
+
+std::optional<std::string> decodeGetProjExtState(int apiReturn,
+                                                 const std::string& buffer) {
+    // REAPER returns the length of the stored value; 0 means the key is absent. Guard
+    // both the return AND the buffer: a caller that reused a dirty buffer must not
+    // surface stale bytes as a value when the API reported nothing.
+    if (apiReturn <= 0 || buffer.empty()) return std::nullopt;
+    return buffer;
+}
+
+std::optional<std::string> extractJsonStringField(const std::string& json,
+                                                  const std::string& key) {
+    // Find the member token: "key" followed (after optional whitespace) by ':' then a
+    // quoted string. Scan for each candidate occurrence of the quoted key so a value
+    // that happens to contain the key text can't produce a false match.
+    const std::string needle = "\"" + key + "\"";
+    size_t searchFrom = 0;
+
+    while (true) {
+        const size_t keyPos = json.find(needle, searchFrom);
+        if (keyPos == std::string::npos) return std::nullopt;
+
+        size_t i = keyPos + needle.size();
+        searchFrom = i;  // next candidate starts after this key token
+
+        // Skip whitespace to the ':'.
+        while (i < json.size() &&
+               (json[i] == ' ' || json[i] == '\t' || json[i] == '\n' ||
+                json[i] == '\r')) {
+            ++i;
+        }
+        if (i >= json.size() || json[i] != ':') continue;  // not a member — keep looking
+        ++i;
+
+        // Skip whitespace to the value.
+        while (i < json.size() &&
+               (json[i] == ' ' || json[i] == '\t' || json[i] == '\n' ||
+                json[i] == '\r')) {
+            ++i;
+        }
+        if (i >= json.size() || json[i] != '"') return std::nullopt;  // value not a string
+        ++i;
+
+        // Read the string body, honoring the common JSON escapes.
+        std::string out;
+        while (i < json.size()) {
+            const char c = json[i];
+            if (c == '\\') {
+                if (i + 1 >= json.size()) return std::nullopt;  // dangling escape
+                const char e = json[i + 1];
+                switch (e) {
+                    case '"': out.push_back('"'); break;
+                    case '\\': out.push_back('\\'); break;
+                    case '/': out.push_back('/'); break;
+                    case 'n': out.push_back('\n'); break;
+                    case 't': out.push_back('\t'); break;
+                    case 'r': out.push_back('\r'); break;
+                    case 'b': out.push_back('\b'); break;
+                    case 'f': out.push_back('\f'); break;
+                    default: out.push_back(e); break;  // pass through unknown escapes
+                }
+                i += 2;
+                continue;
+            }
+            if (c == '"') return out;  // closing quote — done
+            out.push_back(c);
+            ++i;
+        }
+        return std::nullopt;  // unterminated string
+    }
+}
+
+}  // namespace reasampler::vst