fix(playback): report software scaler results and scope smoke ring diagnostics

The software-renderer fallback now reports which scaler options mpv
accepted and which it rejected (with mpv's error), instead of always
printing success. The smoke's black-canvas diagnostics read only the
failed session's rings (<sessionId>-g<N>), not every impv-fc ring in
/dev/shm.

Addresses Greptile review feedback.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/electron-backend-e2e/src/embedded-mpv-frame-copy-packaged-diagnostics.ts

@@ -20,7 +20,6 @@ import {
  */
 
 const SHM_DIRECTORY = '/dev/shm';
-const SHM_PREFIX = 'impv-fc-';
 const FRAME_SHM_MAGIC = 0x564d5046;
 const FRAME_SHM_RING_SLOTS = 3;
 const HEADER_BYTES = 56 + FRAME_SHM_RING_SLOTS * 16;
@@ -103,10 +102,16 @@ export function describeFrameRing(
     };
 }
 
-export function describeFrameRings(): FrameRingDiagnostics[] | string {
+/**
+ * The helper names its rings `/<sessionId>-g<generation>`, so the prefix
+ * keeps stale or concurrent sessions out of the report.
+ */
+export function describeFrameRings(
+    sessionId: string
+): FrameRingDiagnostics[] | string {
     try {
         return readdirSync(SHM_DIRECTORY)
-            .filter((entry) => entry.startsWith(SHM_PREFIX))
+            .filter((entry) => entry.startsWith(`${sessionId}-g`))
             .map((entry) =>
                 describeFrameRing(
                     entry,
@@ -146,7 +151,7 @@ export async function expectRenderedFrame(
                 videoHeight: session.videoHeight,
                 stats: session.stats,
             },
-            frameRings: describeFrameRings(),
+            frameRings: describeFrameRings(sessionId),
         };
         const body = JSON.stringify(diagnostics, null, 2);
         console.log(`[frame-copy smoke diagnostics] ${body}`);

apps/electron-backend/native/helper/mpv_frame_helper.cpp

@@ -803,12 +803,19 @@ void applySoftwareRendererOptions(const std::set<std::string>& sessionKeys) {
         {"dscale", "bilinear"},
         {"sigmoid-upscaling", "no"},
     };
+    std::string applied;
+    std::string failed;
     for (const auto& [key, value] : kOptions) {
-        if (sessionKeys.count(key) == 0) {
-            mpv_set_property_string(g_state.mpv, key, value);
+        if (sessionKeys.count(key) != 0) continue;
+        const int result = mpv_set_property_string(g_state.mpv, key, value);
+        std::string& list = result < 0 ? failed : applied;
+        list += (list.empty() ? "" : ", ") + std::string(key) + "=" + value;
+        if (result < 0) {
+            list += std::string(" (") + mpv_error_string(result) + ")";
         }
     }
-    std::fprintf(stderr, "software renderer: using bilinear scalers\n");
+    std::fprintf(stderr, "software renderer: applied [%s]; failed [%s]\n",
+                 applied.c_str(), failed.c_str());
 }
 
 struct HelperArgs {

docs/development/electron-debugging.md

@@ -93,7 +93,7 @@ The Linux portable build uploads `packaged-frame-copy-smoke` reports and traces
 even when the smoke fails. Check the paused-frame screenshot and trace before
 classifying a zero rendered-frame signal as an infrastructure flake. A zero
 signal also attaches `frame-copy-diagnostics` (the session's stream stats,
-including mpv's drop counter, and each helper ring read from `/dev/shm`) and
+including mpv's drop counter, and its helper rings read from `/dev/shm`) and
 `mpv-log`, the session's verbose mpv log. A `latestSeq` of 0 means the helper
 never published; a `latestFrameSignal` of 0 means mpv rendered black; a
 visible ring frame behind a black canvas points at the preload pump.