feat(rp): honor Arduino-Pico Bluetooth stack menu

[zackees]

Context

FastLED #3832 requires an RP2350W BLE GATT transport. Arduino-Pico 4.5.3 exposes the required btstack and CYW43 support only through its Bluetooth stack menu. For
pipico2w, �oards.txt maps ipbtstack=ipv4btcble to libipv4-bt.a plus -DENABLE_CLASSIC=1 -DENABLE_BLE=1.

fbuild's RP orchestrator loads board menu properties and applies libpicowdefs, but
p_support_objects() always links libipv4.a. It also does not fold RP environment �uild_flags into its compiler configuration. Consequently fbuild cannot select the core Bluetooth archive required for a legitimate RP2350W BLE build.

Proposal

Honor the board's ipbtstack menu selection for RP Arduino-Pico builds. Resolve the selected �uild.libpicow archive and matching �uild.libpicowdefs from board properties, use that archive in support link inputs, and preserve the default IPv4 selection for existing projects.

Acceptance criteria

• Add a focused RED test using an RP Pico W fixture with �oard_build.ipbtstack = ipv4btcble; it must initially show the Bluetooth archive/defines are not selected.
• Make the test GREEN: build inputs include libipv4-bt.a, ENABLE_CLASSIC=1, and ENABLE_BLE=1.
• Keep the default RP network selection on libipv4.a unchanged.
• Produce an RP2350W build whose compile database/link inputs demonstrate the selected Bluetooth variant.
• Run the focused fbuild test and relevant RP build regression tests.

Related issues

FastLED/FastLED#3832 and FastLED/FastLED#3861.