fix(rp2040): match UCRT UF2 handle sharing

[zackees]

Summary

• match Arduino-Pico/CPython's Windows UCRT _SH_DENYNO semantics for NEW.UF2 by allowing shared read/write but not Rust's extra delete sharing
• prevent removable-volume software from deleting or replacing the transfer target while fbuild owns it
• correct Windows recovery text: a blank/invalid-flash Pico returns to ROM boot after a normal fresh enumeration, so no physical BOOTSEL press is required

Source evidence

• Arduino-Pico 4.5.3 uf2conv.py uses open(name, "wb") and one large buffered write
• CPython 3.7 reaches UCRT _wopen; UCRT maps _SH_DENYNO to FILE_SHARE_READ | FILE_SHARE_WRITE only
• Rust OpenOptions additionally enables FILE_SHARE_DELETE by default

Validation

• RED test first: missing parity helper
• 181 fbuild-deploy tests passed; 8 hardware-only tests ignored
• 25 focused RP2040 tests passed
• strict clippy, rustfmt, and diff checks passed
• two high-reasoning independent review passes: approved
• attached-board retry remained safely fail-closed with Windows error 1006 and no false eject/CDC success

Supports #1049 and #1040.

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📥 Commits

Reviewing files that changed from the base of the PR and between 5e047a9 and e5c407f.

📒 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-ucrt-share-mode

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