fix(dfu): Isolate BLE DFU fallback orchestration

Move DFU protocol ordering and session retry budget selection into a fallback coordinator so SecureDfuHandler delegates protocol fallback decisions instead of owning them directly.

Replace the mutable transfer progression flag with an explicit upload result that reports whether a protocol session was engaged before a failure. This keeps alternate-protocol fallback limited to pre-engagement failures while preserving Legacy retry handling, stale-session reset priming, and NonCancellable transport cleanup.

Author: jeremiah-k