fix: add stock-ROM picotool fallback for RP-series deploy

[zackees]

Continues #1049 and closes the stock-board transport gap exposed by attached Pico 1 acceptance.

The documented Arduino-Pico default remains UF2 mass-storage. fbuild keeps that as the primary zero-driver path, but if the host rejects the synthetic RPI-RP2 FAT write, it now falls back to the already managed/pinned picotool against the stock ROM BOOTSEL interface. This does not require pre-flashed firmware and never searches PATH.

If both transports fail, the error preserves both diagnostics and explains the official Windows RP2 Boot Interface 1 WinUSB host-driver option without implying that the board needs special firmware. No VID/PID constants were added.

Validated with:

• soldr cargo test -p fbuild-deploy rp2040 --lib (21 passed)
• soldr cargo clippy -p fbuild-deploy --all-targets -- -D warnings
• soldr cargo fmt --all -- --check
• git diff --check

Hardware observation: the attached stock Pico enumerates and exposes readable INFO_UF2.TXT, but this Windows session rejects the synthetic FAT write with OS error 1392 and has no driver bound to RP2 Boot Interface 1. The combined fallback diagnostic is the required behavior until either the volume is freshly enumerated or the host WinUSB driver is installed.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7c45a32 and 6e640a4.

📒 Files selected for processing (2)

• crates/fbuild-deploy/src/rp2040.rs
• crates/fbuild-deploy/src/rp2040_picotool.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/rp2040-stock-transport-fallback

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