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/timebased_widget_test.cpp

@@ -38,11 +38,22 @@ TEST(SeekableWidget, SetScrollAdjustsScrollBar)
 	widget.SetScroll(0);
 	EXPECT_EQ(widget.GetScroll(), 0);
 
-	int max_scroll = widget.horizontalScrollBar()->maximum();
-	if (max_scroll > 0) {
-		widget.SetScroll(max_scroll);
-		EXPECT_EQ(widget.GetScroll(), max_scroll);
-	}
+	// Give the scene a deterministic length much wider than the viewport so
+	// the horizontal scrollbar gains a non-zero range (60 seconds at
+	// 100 px/second)
+	widget.SetTimebase(olive::rational(1, 30));
+	widget.SetScale(100.0);
+	widget.SetEndTime(olive::rational(60));
+
+	const int max_scroll = widget.horizontalScrollBar()->maximum();
+	ASSERT_GT(max_scroll, 0);
+
+	widget.SetScroll(max_scroll);
+	EXPECT_EQ(widget.GetScroll(), max_scroll);
+
+	// Values beyond the range clamp to the scrollbar's maximum
+	widget.SetScroll(max_scroll + 1000);
+	EXPECT_EQ(widget.GetScroll(), max_scroll);
 }
 
 TEST(SeekableWidget, SetMarkersAndWorkAreaAreReflected)