Refactor: FirmwareService._enrich_firmware_file mutates input dict

[danielcopper]

Problem

FirmwareService._enrich_firmware_file mutates the input dict directly. This is inconsistent with the domain layer pattern where functions return new data without side effects.

While this is internal (no external impact), it's a code smell that goes against the project's established patterns.

Fix

Return a new dict instead of mutating the input, or document the mutation explicitly.

Priority

P0 — consistency with architecture patterns

Area

Architecture, Firmware