Step 7.1: player plumbing (PSRAM ring -> AES67 TX), test tone mode

- main/audio_ring: SPSC ring of interleaved int32 frames in PSRAM
  (4 s at 48 kHz stereo), lock-free with acquire/release counters; the
  TX pull callback never blocks.
- main/player: source selection from source.mode and the pull callback
  for aes67_tx. tone -> the core's PTP-phased 1 kHz tone; off -> silence;
  hls/spotify -> ring with 1 s prefill (silence while buffering is not an
  underrun; running dry is, and prefills again). Status fields
  active_source, source_state, spotify_state, buffer_ms. source config
  applies live.
- New source.mode "tone" (validation, UI dropdown, doc) for commissioning.
- Temporary 440 Hz producer in hls mode (until the HLS player exists).
- Verified: tone 999.7 Hz -18 dBFS; off silent; hls 440.0 Hz with max
  sample step 60816 (ideal sine 60825, i.e. no discontinuities), buffer
  3983 ms, 0 underruns; spotify silent/not implemented.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

main/audio_ring.c

@@ -0,0 +1,76 @@
+#include "audio_ring.h"
+
+#include <string.h>
+
+#include "esp_heap_caps.h"
+
+static int32_t *s_buf;
+static size_t s_cap;                // frames
+static int s_ch;
+// Monotonic frame counters; level = wr - rd. Written by one side each (acquire/release).
+static uint32_t s_wr, s_rd;
+
+esp_err_t audio_ring_init(size_t capacity_frames, int channels)
+{
+    s_buf = heap_caps_malloc(capacity_frames * channels * sizeof(int32_t), MALLOC_CAP_SPIRAM);
+    if (!s_buf) {
+        return ESP_ERR_NO_MEM;
+    }
+    s_cap = capacity_frames;
+    s_ch = channels;
+    return ESP_OK;
+}
+
+size_t audio_ring_level(void)
+{
+    return __atomic_load_n(&s_wr, __ATOMIC_ACQUIRE) - __atomic_load_n(&s_rd, __ATOMIC_ACQUIRE);
+}
+
+size_t audio_ring_space(void)
+{
+    return s_cap - audio_ring_level();
+}
+
+// Copy n frames between the ring (starting at frame index pos) and a linear buffer.
+static void copy(int32_t *ring_to_lin, const int32_t *lin_to_ring, uint32_t pos, size_t n)
+{
+    size_t start = pos % s_cap;
+    size_t first = n < s_cap - start ? n : s_cap - start;
+    size_t fb = first * s_ch * sizeof(int32_t), rb = (n - first) * s_ch * sizeof(int32_t);
+    if (ring_to_lin) {
+        memcpy(ring_to_lin, s_buf + start * s_ch, fb);
+        memcpy(ring_to_lin + first * s_ch, s_buf, rb);
+    } else {
+        memcpy(s_buf + start * s_ch, lin_to_ring, fb);
+        memcpy(s_buf, lin_to_ring + first * s_ch, rb);
+    }
+}
+
+size_t audio_ring_write(const int32_t *frames, size_t n)
+{
+    uint32_t wr = __atomic_load_n(&s_wr, __ATOMIC_RELAXED);
+    size_t space = s_cap - (wr - __atomic_load_n(&s_rd, __ATOMIC_ACQUIRE));
+    n = n < space ? n : space;
+    if (n) {
+        copy(NULL, frames, wr, n);
+        __atomic_store_n(&s_wr, wr + n, __ATOMIC_RELEASE);
+    }
+    return n;
+}
+
+size_t audio_ring_read(int32_t *frames, size_t n)
+{
+    uint32_t rd = __atomic_load_n(&s_rd, __ATOMIC_RELAXED);
+    size_t level = __atomic_load_n(&s_wr, __ATOMIC_ACQUIRE) - rd;
+    n = n < level ? n : level;
+    if (n) {
+        copy(frames, NULL, rd, n);
+        __atomic_store_n(&s_rd, rd + n, __ATOMIC_RELEASE);
+    }
+    return n;
+}
+
+void audio_ring_flush(void)
+{
+    __atomic_store_n(&s_rd, __atomic_load_n(&s_wr, __ATOMIC_ACQUIRE), __ATOMIC_RELEASE);
+}