Add items_waiting/is_empty introspection to ring buffers

[kahrendt]

What

Exposes two introspection methods on the ring buffer types:

• SpscRingBuffer::items_waiting() / is_empty(): count of committed items written but not yet received by the consumer, on both the ESP (FreeRTOS vRingbufferGetInfo) and host implementations. The host implementation gains an items_waiting_ counter (incremented on commit, decremented on receive) and mtx_ is made mutable so the const accessors can lock.
• AudioRingBuffer::chunks_waiting() / is_empty(): thin wrappers delegating to the underlying SPSC ring buffer.

Why

These accessors let the consumer query how much audio is queued so it can detect an empty/near-empty buffer and stuff zeros to prevent underflow to prevent audible glitches at the start of playback.

Notes for reviewers

• No behavioral change to existing code — these are purely additive read-only accessors.
• The host counter is maintained under the existing buffer mutex; the ESP path reads directly from the FreeRTOS ring buffer info, so no extra bookkeeping is needed there.

[Copilot]

Pull request overview

This PR adds lightweight introspection APIs to the ring buffer abstractions so consumers can query whether buffered audio (or other items) are pending and how many are queued, enabling underflow-avoidance logic.

Changes:

• Add SpscRingBuffer::items_waiting() and SpscRingBuffer::is_empty() for both ESP (via vRingbufferGetInfo) and host (via a new items_waiting_ counter).
• Track items_waiting_ in the host implementation (increment on commit, decrement on receive) and make mtx_ mutable to allow locking in const accessors.
• Add SendspinAudioRingBuffer::chunks_waiting() and SendspinAudioRingBuffer::is_empty() as thin wrappers over the underlying SPSC ring buffer.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/platform/spsc_ring_buffer.h	Adds new introspection methods; host implementation adds items_waiting_ counter and updates synchronization to support const accessors.
src/audio_ring_buffer.h	Exposes audio-specific wrappers for the underlying ring buffer introspection.

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