fix: initialize complete backup checksum image

[PhysicistJohn]

Problem

The RTC backup checksum covers every byte of backup_t except the checksum byte itself. That range includes the packed dummy2 reserved byte, but the stack-local structure is populated field-by-field and never initializes it.

The checksum can therefore incorporate indeterminate stack data and cause otherwise valid analyzer settings to be rejected after reset.

Change

Zero-initialize the complete packed backup image before assigning its persisted fields. This makes the reserved byte and resulting checksum deterministic without changing the backup layout or checksum algorithm.

Verification

• F072 and F303 firmware targets build successfully with Arm GNU 11.3.Rel1.
• The implementation was exercised in the ChibiOS RC5 candidate with a deterministic reserved byte.
• On STM32F303 hardware, configured frequency and attenuation sentinels survived the tested warm-reset path and the post-reset checksum was accepted.

Scope and limitations

• No storage layout, field encoding, RF, UI, or protocol behavior changes.
• Physical warm-reset retention is proven.
• Retention of a before/after sentinel across complete removal of power was not measured and is not claimed by this PR.