fix(rp): ingest common Arduino-Pico platform definitions

[zackees]

Reproduction

A clean FastLED RP2350W AutoResearch build using Arduino-Pico 5.7.0 fails in framework sources because fbuild omits definitions from lib/platform_def.txt.

Observed failures, in order:

• PICO_CYW43_ARCH_HEADER missing in lwip_wrap.cpp
• CYW43_PIO_CLOCK_DIV_DYNAMIC missing in cyw43_wrappers.cpp

Arduino-Pico's ools/platformio-build.py ingests both:

• lib/platform_def.txt
• lib//platform_def.txt

fbuild's
p_manifest_define_files() currently returns only the chip-family file. The common file contains the missing CYW43 and SDK definitions.

Expected

The RP orchestrator consumes both common and family framework definition manifests, matching Arduino-Pico's PlatformIO integration.

Acceptance

• Focused test proves common definitions are applied before family definitions.
• Clean Arduino-Pico 5.7.0 RP2350W build reaches link without consumer-side copies of framework definitions.
• Existing RP orchestrator tests pass.

Coordinated with FastLED/FastLED#3899.