fix(mesh): don't evict ACKs when a background-priority packet arrives full

[DatanoiseTV]

fromRadioQueue is a plain FIFO of size 4. When it fills, the prior
behaviour was to drop the oldest packet — usually a routing ACK or
reply — to make room for whatever just arrived, even if the new packet
was BACKGROUND priority. Reverse that for the low-priority case: if
the incoming packet's priority is at or below BACKGROUND, drop it
instead. Higher-priority arrivals still evict to fit. PointerQueue
doesn't expose iteration, so a full priority-queue refactor would be a
larger change; this addresses the worst case.

Split out from #10424 per @thebentern's request — single-concern PR.

Build verification

pio run -e t-deck-tft succeeds, no new warnings.

Attestations

• I have tested that my proposed changes behave as described — review/static-analysis only, not on-air.
• On-hardware testing requested from community: build-verified t-deck-tft only.

[Copilot]

Pull request overview

This PR improves receive-side routing reliability by changing Router::enqueueReceivedMessage() to avoid evicting already-queued packets (often ACKs/routing replies) when fromRadioQueue is full and the newly arrived packet is low priority (≤ BACKGROUND). This targets a worst-case behavior where a burst of low-priority traffic could displace higher-value control-plane packets.

Changes:

• Drop incoming packets with priority ≤ BACKGROUND when fromRadioQueue is full, instead of evicting the oldest queued packet.
• Preserve the prior “evict oldest until it fits” behavior for higher-priority incoming packets.

[GUVWAF]

Are you observing that the FIFO ever gets full?
This should really not happen: it means we're too slow handling incoming packets. If we're really seeing this (often), IMO we should increase the FIFO size rather than throwing away the "least important one".