[SEC] Redact or shorten MAC addresses in tracing spans

[wkusnierczyk]

Context

PR #99 added `#[instrument]` to all `GoveeBackend` trait methods in both backends, with `device = %id` as a span field. `DeviceId::Display` emits the raw MAC address (e.g. `AA:BB:CC:DD:EE:FF`).

This means every backend operation logs the full MAC in structured tracing spans. Depending on the subscriber (file logger, syslog, log aggregator), MACs may appear in logs shipped off the host.

Affected locations

• `src/backend/cloud.rs` — all `#[instrument]` attrs with `device = %id`
• `src/backend/local.rs` — all `#[instrument]` attrs with `device = %id`

Options

1. Omit the device field from spans entirely (loses observability).
2. Shorten to last 3 octets or a hash prefix for correlation without full disclosure.
3. Redacting wrapper type — a newtype over `DeviceId` whose `Display` masks the MAC, used only in tracing field position.

Option 3 preserves correlation (same partial ID across spans for the same device) while not logging the full address.

Notes

• Raised in PR M07 Wave 3: Reliability, observability, and performance #99 review (comments r2966476073, r2966476177).
• Device names from the Govee cloud API are also untrusted/external (RT-M07-05) and appear in spans — that is a separate concern already noted in the tracing doc comments.