fix: compile BLE split DFU

[colonelpanic8]

Summary

• provide explicit initial values for split GATT characteristics when DFU enlarges the message buffer
• pass the existing UpdatePolicy through the BLE central call chain

Why

Enabling BLE and dfu_split together exposes two pre-existing compile failures:

• DFU firmware chunks increase SPLIT_MESSAGE_MAX_SIZE to 271 bytes, but TrouBLE's derive-generated default initialization does not work for [u8; 271]
• dfu_split adds an UpdatePolicy argument to PeripheralManager::new, but the BLE path did not forward it

This PR contains only those compile-enablement changes. The notification-readiness fix is stacked separately in #2.

Validation

• RUST_MIN_STACK=16777216 cargo +1.97.0 clippy --bin peripheral --features rmk/dfu_rp,rmk/dfu_split -- -D warnings in examples/use_rust/pi_pico_w_ble_split
• sh scripts/format_all.sh --touched
• git diff --check

Stacked on #3.