Fix Pico builds with the io umbrella manifest

[HipsterBrown]

On v8.0.0, including the umbrella $(MODDABLE)/modules/io/manifest.json in a project targeted at -p pico fails to build for the original RP2040 Raspberry Pi Pico (no Wi-Fi). This worked in v7.2.1.

Repro:

manifest.json:

{
  "include": [
    "$(MODDABLE)/examples/manifest_base.json",
    "$(MODDABLE)/modules/io/manifest.json"
  ],
  "modules": { "*": "./main" }
}

main.js uses only embedded:io/pwm + timer.

mcconfig -d -m -p pico

Errors seen:

1. fatal error: pico/cyw43_arch.h: No such file or directory (modNet.c)
2. fatal error: lwip/tcp.h: No such file or directory (modResolve.c)
3. MODDEF_AUDIOOUT_I2S_DATAOUT_PIN undeclared / MODDEF_AUDIOOUT_I2S_BCK_PIN undeclared / #error LR_PIN must be BCK_PIN + 1 (audioout-pico.c)
4. initialization of 'int16_t *' from incompatible pointer type 'uint8_t *' (audioout-pico.c:579)

Fixes:

1. modules/io/manifest.json — bare pico platform should not force networking

Before, the bare pico platform unconditionally included manifests/pico/manifest_net.json, which pulls examples/manifest_net.json and compiles modNet.c / modResolve.c. Those require pico/cyw43_arch.h and lwip/* — headers that only exist when the build is configured for the CYW43 radio (Pico W / Pico 2 W). This has been resolved by pointing the default pico target to the original $(IO)/manifests/pico/manifest.json and the compatible wireless boards to the new manifest_net.json for the pico. This seems to be a unique case for the pico since the other platforms are "all or nothing" when it comes to wireless connectivity.

2. modules/io/audioout/pico/audioout-pico.c — guard MODDEF_AUDIOOUT_I2S_* defines

The pico audioout source unconditionally references MODDEF_AUDIOOUT_I2S_DATAOUT_PIN, MODDEF_AUDIOOUT_I2S_BCK_PIN, MODDEF_AUDIOOUT_I2S_LR_PIN, and MODDEF_AUDIOOUT_I2S_BITSPERSAMPLE, but no manifest provides defaults — so any project pulling in audioout without explicit pin overrides fails to compile.

The esp32 audioout driver already follows the #ifndef-with-default pattern, so this PR applies it to pico, defaulting to the pin macros pico-extras already exposes. Pico-extras conventionally sets PICO_AUDIO_I2S_DATA_PIN=9 and PICO_AUDIO_I2S_CLOCK_PIN_BASE=10, matching the Pimoroni Pico Audio Pack pinout. Existing manifests that already define these macros are unaffected.

3. modules/io/audioout/pico/audioout-pico.c:579 — pointer-type mismatch in doWrite

buffer->buffer->bytes is uint8_t *; assigning into int16_t * is an -Wincompatible-pointer-types error under recent GCC. The arithmetic is intentionally byte-offset, so the fix is applying an explicit cast.

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I have tested these fixes only with my local test program on the Pico. I don't have a Pico W or Pico 2W to test with at the moment.