feat(serial): HandoffTiming per BoardFamily + matrix doc (#691)

[zackees]

#691. Pins LPC845-BRK numbers from FastLED/FastLED#3339 plus observed ESP/Teensy/RP2040/Arduino values so Deployer::post_deploy_recovery (#605) can stop hardcoding magic numbers per-deployer.

• HandoffTiming struct (post_reset_settle_ms, boot_drain_ms, port_reappear_timeout_ms, open_retry_count)
• BoardFamily::handoff_timing() — concrete table
• 4 new tests pinning the LPC numbers + ESP/1200-bps/Arduino invariants
• Doc table in docs/usb-cdc-control-line-matrix.md

Out of scope (follow-up): each Deployer reading the settings instead of inline magic numbers; FastLED Python-side adoption via shared helper.

Closes #691.

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📒 Files selected for processing (2)

• crates/fbuild-serial/src/boards.rs
• docs/usb-cdc-control-line-matrix.md

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