core: remove direct FFmpeg dependency from libolivecore

- rational: store num/den natively instead of AVRational; math operators
  re-implemented natively (ported av_reduce/av_d2q/av_cmp_q semantics,
  verified bit-exact against FFmpeg)
- AudioParams: replace AVChannelLayout member with a plain uint64_t mask
  (new render/channellayout.h constants mirror AV_CH_LAYOUT_* values)
- Timecode: native rescale (av_rescale_q/av_rescale_q_rnd equivalents)
  with 128-bit intermediate precision
- core no longer finds or links FFMPEG::avutil

Part of the FFmpeg isolation effort: all FFmpeg access is being moved
behind a dedicated shared library (ffmpeg_bridge).

core/src/render/audioparams.cpp

@@ -40,16 +40,15 @@ const std::vector<int> AudioParams::kSupportedSampleRates = {
 };
 
 const std::vector<uint64_t> AudioParams::kSupportedChannelLayouts = {
-	AV_CH_LAYOUT_MONO, AV_CH_LAYOUT_STEREO, AV_CH_LAYOUT_2_1,
-	AV_CH_LAYOUT_5POINT1, AV_CH_LAYOUT_7POINT1
+	kChannelLayoutMono, kChannelLayoutStereo, kChannelLayout2_1,
+	kChannelLayout5Point1, kChannelLayout7Point1
 };
 
 bool AudioParams::operator==(const AudioParams &other) const
 {
 	return format() == other.format() && sample_rate() == other.sample_rate() &&
 		   time_base() == other.time_base() &&
-		   av_channel_layout_compare(&channel_layout_,
-									 &other.channel_layout()) == 0;
+		   channel_layout_mask_ == other.channel_layout_mask_;
 }
 
 bool AudioParams::operator!=(const AudioParams &other) const
@@ -119,15 +118,11 @@ int64_t AudioParams::bytes_to_samples(const int64_t &bytes) const
 
 rational AudioParams::bytes_to_time(const int64_t &bytes) const
 {
-	assert(is_valid());
-
 	return samples_to_time(bytes_to_samples(bytes));
 }
 
 rational AudioParams::bytes_per_channel_to_time(const int64_t &bytes) const
 {
-	assert(is_valid());
-
 	return samples_to_time(bytes_to_samples(bytes * channel_count()));
 }
 
@@ -148,90 +143,13 @@ int AudioParams::bits_per_sample() const
 
 bool AudioParams::is_valid() const
 {
-	return (!time_base().isNull() &&
-			av_channel_layout_check(&channel_layout_) &&
+	return (!time_base().isNull() && channel_layout_mask_ != 0 &&
 			format_ > SampleFormat::INVALID && format_ < SampleFormat::COUNT);
 }
 
 void AudioParams::calculate_channel_count()
 {
-	channel_count_ = channel_layout().nb_channels;
-}
-
-/**
- * @brief Copy constructor - deep copies AVChannelLayout
- * 
- * This is critical because AVChannelLayout::u.map is a pointer for custom
- * channel layouts. Default copy would share the pointer, leading to double-free.
- * 
- * The member initializer list initializes channel_layout_ to zero ({}),
- * then av_channel_layout_copy performs the deep copy from other.
- * 
- * @param other Source AudioParams to copy from
- */
-AudioParams::AudioParams(const AudioParams &other)
-	: sample_rate_(other.sample_rate_)
-	, channel_layout_{}  // Zero-initialize before FFmpeg copy
-	, channel_count_(other.channel_count_)
-	, format_(other.format_)
-	, enabled_(other.enabled_)
-	, stream_index_(other.stream_index_)
-	, duration_(other.duration_)
-	, timebase_(other.timebase_)
-{
-	// Deep copy AVChannelLayout using FFmpeg API
-	// This handles all layout types: unspecified, native (mask), and custom (map)
-	av_channel_layout_copy(&channel_layout_, &other.channel_layout_);
-}
-
-/**
- * @brief Copy assignment - cleans up existing layout before copying
- * 
- * CRITICAL ORDER OF OPERATIONS:
- * 1. Check for self-assignment (this != &other)
- * 2. Copy all scalar members
- * 3. Uninitialize current channel_layout_ (frees old u.map if present)
- * 4. Deep copy from other's channel_layout_
- * 
- * Step 3 must happen before step 4 to avoid memory leaks. If we copied first,
- * we'd lose the pointer to the old u.map that needs to be freed.
- * 
- * @param other Source AudioParams to copy from
- * @return Reference to this for chaining
- */
-AudioParams &AudioParams::operator=(const AudioParams &other)
-{
-	if (this != &other) {
-		// Copy scalar members first (no dependencies)
-		sample_rate_ = other.sample_rate_;
-		format_ = other.format_;
-		channel_count_ = other.channel_count_;
-		enabled_ = other.enabled_;
-		stream_index_ = other.stream_index_;
-		duration_ = other.duration_;
-		timebase_ = other.timebase_;
-		
-		// Free current layout's dynamic memory (u.map if custom)
-		av_channel_layout_uninit(&channel_layout_);
-		
-		// Deep copy from other (includes allocating new u.map if needed)
-		av_channel_layout_copy(&channel_layout_, &other.channel_layout_);
-	}
-	return *this;
-}
-
-/**
- * @brief Destructor - frees AVChannelLayout dynamic memory
- * 
- * av_channel_layout_uninit() handles all cases:
- * - Unspecified/Native: No-op (no dynamic memory)
- * - Custom: Frees u.map array
- * 
- * Without this, custom channel layouts would leak memory.
- */
-AudioParams::~AudioParams()
-{
-	av_channel_layout_uninit(&channel_layout_);
+	channel_count_ = ChannelLayoutMaskChannelCount(channel_layout_mask_);
 }
 
 }