PlanarView: Add channel stride and subView

[eyalamirmusic]

What changed

PlanarView gains a channel stride: channel c now starts c * channelStride elements after channel 0. The stride defaults to the channel length, so a view built with the existing constructors is still one contiguous block. A larger stride leaves gaps between channels, which is what a range of samples inside a larger planar block looks like.

Why

To represent sub-block ranges of a multi-channel planar buffer. A polyphonic synth base in ESeq needs to hand each voice a zero-based view of just the samples it renders (e.g. from a note-on partway through the block), without copying. Existing callers keep identical behaviour, because the default stride equals the sample count.

API added

• PlanarView(T* data, SizeType numChannels, SizeType numSamples, SizeType channelStride), plus a matching deduction guide. Asserts channelStride >= numSamples when there is more than one channel.
• int getChannelStride() const
• bool isContiguous() const: true when empty, single-channel, or channelStride == numSamples.
• PlanarView subView(SizeType offset, SizeType count = toEnd) const: samples [offset, offset + count) of every channel, keeping the stride. Offset and count are clamped to the channel length.
• static constexpr int toEnd (forwarded from Span<T>::toEnd).

Behaviour changes inside the class

• getChannel, operator[] and PlanarIterator offset by the stride, not the sample count.
• The const-converting constructor carries the stride over.
• fill now fills channel by channel, so it leaves the gaps between channels alone.
• flat() asserts isContiguous().
• getNumElements() still returns the logical count (channels × samples) and excludes the gaps.
• sizeof(PlanarView) grows by one int.

Tests

• Renamed PlanarView.is_a_pointer_and_two_ints to PlanarView.is_a_pointer_and_three_ints. Its static_assert now compares against a struct of a pointer and three ints, so it accounts for padding to the pointer's alignment.
• New tests cover: the strided constructor, deduction with a stride, subView offsets, iterating a strided view, fill on a sub-view leaving the gaps alone, const conversion keeping the stride, isContiguous cases, subView clamping and empty results, flat() on an empty strided sub-view, and subView at compile time.

Test results

488/488 passing in this repo's own build.