fix(rp2040): accept reset disconnect after BOOTSEL

[zackees]

Summary

• match Arduino-Pico's 1200-bps reset ordering by holding DTR asserted for 100 ms before deasserting and switching baud
• treat a Windows CDC disconnect error as provisional, then accept it only when exactly one verified RP2040 BOOTSEL volume appears
• preserve the reset diagnostic when BOOTSEL does not appear

Fixes the reset/disconnect race found during FastLED/FastLED#3626 bring-up. Related: #1040.

Validation

• soldr cargo test -p fbuild-deploy — 189 passed, 8 ignored
• soldr cargo clippy -p fbuild-deploy --all-targets -- -D warnings
• genuine Pico 1 COM12 (2E8A:000A, serial 5303284720C4641C): touchless runtime CDC -> BOOTSEL -> UF2 -> runtime CDC
• ten consecutive bash autoresearch rp2040 --rpc-smoke --timeout 120s --skip-lint cycles through editable fbuild: 10/10 passed
• watchdog reset/recovery and post-reset RPC smoke passed on the same board

No production VID/PID literals were added; identity continues to come from the FastLED/boards profile cache.

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Review details

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: dbeb679f-3905-446c-9689-3e5a1530ed81

📥 Commits

Reviewing files that changed from the base of the PR and between 8f7e860 and d44154e.

📒 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-reset-disconnect-race

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