Add stable hashed identity for speed dial actions

src/slic3r/GUI/SpeedDialActionId.hpp

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <cstdint>
+#include <string>
+
+namespace Slic3r { namespace GUI {
+
+// Stable, opaque identity for a speed-dial action, used as the AppConfig key,
+// the JS/registry handle, and the favourite_actions/stats key. FNV-1a 64-bit over
+// the UTF-8 bytes of `plugin_key 0x1f capability`, lowercase 16-char hex.
+//
+// why: this is PERSISTED, so it must be deterministic across runs and platforms.
+// std::hash<std::string> is explicitly NOT usable here (unspecified, per-process).
+// note: 64-bit hash -> collision is astronomically unlikely at the tens-of-actions
+//   scale; upgrade path (128-bit / full-string key) only if that ever changes.
+inline std::string speed_dial_action_id(const std::string& plugin_key, const std::string& capability)
+{
+    constexpr std::uint64_t kOffset = 14695981039346656037ULL; // FNV offset basis (0xcbf29ce484222325)
+    constexpr std::uint64_t kPrime  = 1099511628211ULL;       // FNV prime
+    std::uint64_t h = kOffset;
+    // note: [&], not [&h] - MSVC rejects the constexpr kPrime read inside the lambda
+    // without a default capture mode (error C3493).
+    auto mix = [&](const std::string& s) {
+        for (unsigned char c : s) { h ^= c; h *= kPrime; }
+    };
+    mix(plugin_key);
+    h ^= 0x1f; h *= kPrime;   // 0x1f (unit separator) between the two fields
+    mix(capability);
+
+    static const char* kHex = "0123456789abcdef";
+    std::string out(16, '0');
+    for (int i = 15; i >= 0; --i) { out[i] = kHex[h & 0xf]; h >>= 4; }
+    return out;
+}
+
+}} // namespace Slic3r::GUI