fix(playback): seek Embedded MPV steps relative to mpv's own position

Arrow keys and the ±10 s buttons in the Embedded MPV player advanced only
about a second per press when pressed repeatedly or held. The shortcuts
already asked for 5 s steps, but `EmbeddedMpvCommandRunner.seekBy` turned
each step into an absolute `seek` computed from `session.positionSeconds`,
which is floored to whole seconds, polled every 500 ms (helper snapshots at
most every 250 ms) and not refreshed by the seek reply. Every press inside
that window therefore landed on the same target.

Steps now go through a new `EMBEDDED_MPV_SEEK_BY` IPC / `seekEmbeddedMpvBy`
bridge method that every backend forwards as mpv `seek <delta>
relative+exact`: `seekBy` exports in the macOS addon and the Windows/Linux
`wid` addon (Linux over its JSON IPC socket), and a `seek-by` stdin command
in the frame-copy helper. mpv resolves the delta against its own position
and merges queued relative seeks, so presses accumulate as in mpv itself.
The absolute form survives only as a fallback for a preload without the
method or an addon binary without `seekBy`; the timeline scrub still
commits an absolute target.

Validated with a real mpv 0.39 IPC probe: three relative seeks in a burst
advance +15 s, three absolute seeks from one stale base advance +5 s.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

apps/electron-backend/native/src/embedded_mpv.mm

@@ -2051,6 +2051,50 @@ Napi::Value Seek(const Napi::CallbackInfo& info)
     return env.Undefined();
 }
 
+Napi::Value SeekBy(const Napi::CallbackInfo& info)
+{
+    Napi::Env env = info.Env();
+    if (info.Length() < 2 || !info[0].IsString() || !info[1].IsNumber()) {
+        throw Napi::TypeError::New(env, "Expected session id and seek delta.");
+    }
+
+    const std::string sessionId = info[0].As<Napi::String>().Utf8Value();
+    const auto session = getSessionOrThrow(env, sessionId);
+    const auto delta = info[1].As<Napi::Number>().DoubleValue();
+    const std::string deltaValue = std::to_string(delta);
+    // Relative step (arrow keys, ±10 s buttons): mpv resolves the delta
+    // against its own playback position and merges relative seeks that are
+    // still queued, so a burst of presses accumulates instead of collapsing
+    // onto one target computed from the renderer's stale snapshot.
+    const char* command[] = {
+        "seek",
+        deltaValue.c_str(),
+        "relative+exact",
+        nullptr,
+    };
+    const int result = mpv_command_async(
+        session->handle,
+        nextAsyncRequestId(),
+        command
+    );
+
+    if (result < 0) {
+        throw Napi::Error::New(
+            env,
+            std::string("Failed to seek playback: ") +
+                mpv_error_string(result)
+        );
+    }
+
+    {
+        std::lock_guard<std::mutex> lock(session->mutex);
+        session->snapshot.positionSeconds =
+            std::max(0.0, session->snapshot.positionSeconds + delta);
+    }
+
+    return env.Undefined();
+}
+
 Napi::Value SetVolume(const Napi::CallbackInfo& info)
 {
     Napi::Env env = info.Env();
@@ -2540,6 +2584,7 @@ Napi::Object Init(Napi::Env env, Napi::Object exports)
     exports.Set("setBounds", Napi::Function::New(env, SetBounds));
     exports.Set("setPaused", Napi::Function::New(env, SetPaused));
     exports.Set("seek", Napi::Function::New(env, Seek));
+    exports.Set("seekBy", Napi::Function::New(env, SeekBy));
     exports.Set("setVolume", Napi::Function::New(env, SetVolume));
     exports.Set("setAudioTrack", Napi::Function::New(env, SetAudioTrack));
     exports.Set(

apps/electron-backend/native/src/embedded_mpv_wid_common.h

@@ -2000,6 +2000,55 @@ Napi::Value Seek(const Napi::CallbackInfo& info)
     return env.Undefined();
 }
 
+Napi::Value SeekBy(const Napi::CallbackInfo& info)
+{
+    Napi::Env env = info.Env();
+    if (info.Length() < 2 || !info[0].IsString() || !info[1].IsNumber()) {
+        throw Napi::TypeError::New(env, "Expected session id and seek delta.");
+    }
+    const auto session =
+        getSessionOrThrow(env, info[0].As<Napi::String>().Utf8Value());
+    const double delta = info[1].As<Napi::Number>().DoubleValue();
+    const std::string seconds = formatInvariantDouble(delta);
+    // Relative step (arrow keys, ±10 s buttons): mpv resolves the delta
+    // against its own playback position and merges relative seeks that are
+    // still queued, so a burst of presses accumulates instead of collapsing
+    // onto one target computed from the renderer's stale snapshot.
+#ifdef __linux__
+    std::string socketPath;
+    {
+        std::lock_guard<std::mutex> lock(session->mutex);
+        socketPath = session->mpvIpcSocketPath;
+        session->snapshot.positionSeconds =
+            std::max(0.0, session->snapshot.positionSeconds + delta);
+    }
+    if (!socketPath.empty()) {
+        sendLinuxMpvCommand(
+            socketPath,
+            "{\"command\":[\"seek\"," + seconds + ",\"relative+exact\"]}\n"
+        );
+    }
+    return env.Undefined();
+#endif
+    const char* command[] = {
+        "seek", seconds.c_str(), "relative+exact", nullptr
+    };
+    const int result = mpv_command_async(
+        session->handle,
+        nextAsyncRequestId(),
+        command
+    );
+    if (result < 0) {
+        throw Napi::Error::New(env, mpv_error_string(result));
+    }
+    {
+        std::lock_guard<std::mutex> lock(session->mutex);
+        session->snapshot.positionSeconds =
+            std::max(0.0, session->snapshot.positionSeconds + delta);
+    }
+    return env.Undefined();
+}
+
 Napi::Value SetVolume(const Napi::CallbackInfo& info)
 {
     Napi::Env env = info.Env();
@@ -2360,6 +2409,7 @@ Napi::Object Init(Napi::Env env, Napi::Object exports)
     exports.Set("setBounds", Napi::Function::New(env, SetBounds));
     exports.Set("setPaused", Napi::Function::New(env, SetPaused));
     exports.Set("seek", Napi::Function::New(env, Seek));
+    exports.Set("seekBy", Napi::Function::New(env, SeekBy));
     exports.Set("setVolume", Napi::Function::New(env, SetVolume));
     exports.Set("setAudioTrack", Napi::Function::New(env, SetAudioTrack));
 #ifndef __linux__