test: replace fake and duplicate tests with real assertions

Fake tests rewritten to assert real behavior:
- audio_smoke: conversion tests now actually Convert() samples and verify
  output; waveform length/summary assertions tightened to exact values
- plugin_format_conversion: RowBytes/U8ToU16/LoadImageFile now call real
  production code (VideoParams::GetBytesPerPixel, sws scaler, OIIO decode
  of tests/img.png with known pixel values)
- core_color: HSV round trip now verifies fromHsv(toHsv(c)) == c instead
  of comparing toHsv against its own accessors
- core_bezier/node_inputimmediate: expected values replaced with
  independently derived constants instead of re-running the code under test
- common_commandlineparser/common_debug/common_jobtime: capture
  stdout/stderr/qDebug and assert actual output content
- proxy_manager: ProxyFinished test now drives a real proxy job instead of
  emitting the signal itself; proxy_dialog/panel/proxy/preferences/timeruler
  tests assert real widget state
- viewer_smoke/preview_autocacher/render_misc: zero-assertion tests given
  observable-state assertions or removed where nothing is observable

Duplicates removed:
- plugin_smoke_test.cpp: 18 tests duplicated from plugin_paraminstance /
  plugin_support_* / plugin_renderer_readback (751 -> 180 lines)
- module_smoke HumanStrings tests covered precisely by ui_humanstrings_test
- render_misc duplicate kDefaultInterpolation constant check

Removed by policy (skip allowed, never disabled):
- all DISABLED_ prefixes: re-enabled as real offscreen tests or deleted
- ffmpeg_decoder_hw: hardcoded personal path replaced with
  OAK_TEST_HW_DECODE_FILE env var, GTEST_SKIP when unset

Also:
- config_test: restore Config defaults after run (cross-test pollution)
- render_worker_footage: drop /tmp debug-output scaffolding
- previewaudiodevice construction test asserts the real bugfix

tests/gtest/timeline_undo_general_test.cpp

@@ -783,11 +783,49 @@ TEST_F(TimelineUndoGeneralTest, AddDefaultTransitionAddsDualTransition)
 
 TEST_F(TimelineUndoGeneralTest, AddDefaultTransitionEmptyClipListIsHarmless)
 {
+	// A real timeline with two adjacent clips; the empty command must leave
+	// every observable detail of it untouched
+	olive::Sequence *sequence = CreateSequence(project_.get());
+	olive::TrackList *list = sequence->track_list(olive::Track::kVideo);
+	olive::Track *track = CreateTrack(project_.get());
+	olive::ClipBlock *a = CreateClip(project_.get(), olive::core::rational(4));
+	olive::ClipBlock *b = CreateClip(project_.get(), olive::core::rational(4));
+	track->AppendBlock(a);
+	track->AppendBlock(b);
+	AppendTrackToList(list, track);
+	// Layout: a [0,4], b [4,8]
+
 	olive::TimelineAddDefaultTransitionCommand cmd(
 		{}, olive::core::rational(1, 30));
 	EXPECT_EQ(cmd.GetRelevantProject(), nullptr);
 
-	// redo/undo on an empty command must be harmless no-ops
+	// redo on an empty command adds no transitions and changes nothing
 	cmd.redo_now();
+	ASSERT_EQ(track->Blocks().size(), 2);
+	EXPECT_EQ(track->Blocks().at(0), a);
+	EXPECT_EQ(track->Blocks().at(1), b);
+	EXPECT_EQ(a->length(), olive::core::rational(4));
+	EXPECT_EQ(a->media_in(), olive::core::rational(0));
+	EXPECT_EQ(a->in(), olive::core::rational(0));
+	EXPECT_EQ(a->out(), olive::core::rational(4));
+	EXPECT_EQ(b->length(), olive::core::rational(4));
+	EXPECT_EQ(b->media_in(), olive::core::rational(0));
+	EXPECT_EQ(b->in(), olive::core::rational(4));
+	EXPECT_EQ(b->out(), olive::core::rational(8));
+	EXPECT_EQ(track->track_length(), olive::core::rational(8));
+
+	// undo is equally a no-op
 	cmd.undo_now();
+	ASSERT_EQ(track->Blocks().size(), 2);
+	EXPECT_EQ(track->Blocks().at(0), a);
+	EXPECT_EQ(track->Blocks().at(1), b);
+	EXPECT_EQ(a->length(), olive::core::rational(4));
+	EXPECT_EQ(a->media_in(), olive::core::rational(0));
+	EXPECT_EQ(a->in(), olive::core::rational(0));
+	EXPECT_EQ(a->out(), olive::core::rational(4));
+	EXPECT_EQ(b->length(), olive::core::rational(4));
+	EXPECT_EQ(b->media_in(), olive::core::rational(0));
+	EXPECT_EQ(b->in(), olive::core::rational(4));
+	EXPECT_EQ(b->out(), olive::core::rational(8));
+	EXPECT_EQ(track->track_length(), olive::core::rational(8));
 }