wip: park audio-crackle investigation + haptic/OLED defaults + debug tooling

Preserves in-progress work that had been sitting uncommitted in the working
tree (predates this session) so it is protected in git history. This is NOT a
verified/shipped fix — it is parked. Audio path is unverified on hardware and
still carries debug instrumentation.

- src/audio.cpp: drop the 512->480 resampler and retime the 0x36 frame to a
  true 10ms/100Hz grid (SAMPLE_SIZE 64->60, 480-sample buffer) to chase the
  ~45kHz-vs-48kHz speaker underrun theory behind the crackle; dynamic speaker
  sub-report offset; debug printf + Opus-frame-dump instrumentation.
- src/config.cpp: default audio_buffer_length 64->16; auto_haptics_enable
  default 1 (Fallback) -> 0 (Off).
- src/oled.cpp: chunked non-blocking SPI flush (issue #7 OLED-stall angle) and
  a battery-% midpoint display tweak.
- scripts/: audio debug helpers (opus-dump decode, pi audio test, sine ch1/2).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/sine_ch12.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env python3
+"""Send a 440 Hz sine wave on channels 1+2 only (speaker), silence on 3+4 (haptic).
+
+Usage: python3 scripts/sine_ch12.py [seconds]
+"""
+import subprocess, struct, math, sys, re
+
+RATE = 48000
+FREQ = 440
+DURATION = int(sys.argv[1]) if len(sys.argv) > 1 else 5
+CHANNELS = 4
+SAMPLES = RATE * DURATION
+
+# Generate 4-channel S16_LE: sine on ch1+ch2, silence on ch3+ch4
+data = bytearray()
+for i in range(SAMPLES):
+    val = int(6000 * math.sin(2 * math.pi * FREQ * i / RATE))   # ~-15 dBFS (was 32767 / 0 dBFS) — clipping test
+    s = struct.pack('<h', val)
+    data += s + s + b'\x00\x00' + b'\x00\x00'  # L, R, hapL=0, hapR=0
+
+# Auto-detect the dongle's ALSA card — it enumerates as "DualSense Wireless
+# Controller", and the card number shifts across reboots, so never hardcode it.
+def find_dualsense_card():
+    out = subprocess.check_output(['aplay', '-l'], text=True)
+    for line in out.splitlines():
+        m = re.match(r'card (\d+):', line)
+        if m and 'DualSense' in line:
+            return int(m.group(1))
+    return None
+
+card = find_dualsense_card()
+if card is None:
+    print("DualSense dongle not found in `aplay -l` — paired and enumerated?", file=sys.stderr)
+    sys.exit(1)
+device = f'hw:{card},0'
+print(f"Playing to {device} (DualSense dongle)")
+
+proc = subprocess.Popen(
+    ['aplay', '-D', device, '-f', 'S16_LE', '-c', '4', '-r', '48000', '-'],
+    stdin=subprocess.PIPE
+)
+proc.stdin.write(data)
+proc.stdin.close()
+proc.wait()
+print(f"Played {DURATION}s of {FREQ} Hz sine on ch1+ch2 (speaker only)")