fix(rp2040): use Arduino-compatible UF2 write semantics

[zackees]

Summary

• replace Windows-sensitive std::fs::copy with Arduino-Pico-compatible create/truncate + whole-buffer direct write to NEW.UF2
• count reported bytes so an eject-race error is accepted only after the complete artifact and observed BOOTSEL disappearance
• preserve failures for partial writes and for Windows 1006/121/1392 when ROM acceptance is not established
• add actionable Windows 1006/121 diagnostics without filesystem repair, driver changes, raw-disk writes, or copy retries

Validation

• uv run python ci/test.py -p fbuild-deploy: 180 passed, 8 hardware-specific ignored
• soldr cargo test -p fbuild-deploy rp2040: 32 passed
• soldr cargo clippy -p fbuild-deploy --all-targets -- -D warnings
• soldr cargo fmt --all -- --check
• git diff --check
• independent review: approved

Hardware status

The editable build has been exercised on the attached stock Pico 1 (2E8A:0003, RPI-RP2, serial E0C9125B0D9B). The prior whole-file copy failed with Windows error 1006 while the marker remained. A repeated-small-write experiment failed with error 121 and was discarded. The whole-buffer writer is staged, but the current device is behind a Genesys Logic 05E3:0610 USB 2.0 hub and has not yet produced a valid ROM eject/runtime CDC transition. This PR remains draft until a fresh direct-port BOOTSEL run provides the required hardware evidence.

Closes the remaining Phase 2 gap in #1049.
Supports #1040.

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Reviewing files that changed from the base of the PR and between 29adb7a and 1229d73.

📒 Files selected for processing (1)

• crates/fbuild-deploy/src/rp2040.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/rp2040-direct-uf2-write

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[zackees]

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