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/common_debug_test.cpp

@@ -4,7 +4,8 @@
 
 TEST(CommonDebug, DebugHandlerFormatsAllLevels)
 {
-	// Install handler and restore after test
+	// DebugHandler writes "[LEVEL] message" lines to stderr
+	testing::internal::CaptureStderr();
 	QtMessageHandler old = qInstallMessageHandler(olive::DebugHandler);
 
 	qDebug() << "debug message";
@@ -13,4 +14,23 @@ TEST(CommonDebug, DebugHandlerFormatsAllLevels)
 	qCritical() << "critical message";
 
 	qInstallMessageHandler(old);
+	std::string out = testing::internal::GetCapturedStderr();
+
+	// Each level must get its own tag, paired with its message on one line
+	QString output = QString::fromStdString(out);
+	const QStringList lines = output.split('\n');
+	auto has_line = [&lines](const char *tag, const char *text) {
+		for (const QString &line : lines) {
+			if (line.contains(QString::fromLatin1(tag)) &&
+				line.contains(QString::fromLatin1(text))) {
+				return true;
+			}
+		}
+		return false;
+	};
+
+	EXPECT_TRUE(has_line("[DEBUG]", "debug message"));
+	EXPECT_TRUE(has_line("[INFO]", "info message"));
+	EXPECT_TRUE(has_line("[WARNING]", "warning message"));
+	EXPECT_TRUE(has_line("[ERROR]", "critical message"));
 }