fix(agent): scan neighbors per interface with own timeout

[gnacho]

Why

The neighbor scan ran as a single shell loop over every wireless interface with one shared 10 s timeout, and the runner discarded the partial output when the deadline killed the loop. On units whose 5 GHz radio sits on a DFS channel (slow passive scan, e.g. the UniFi 6 Lite / mt7621 from #1338), the loop was killed before the 2.4 GHz interfaces were scanned, so the agent reported no neighbors at all and the channel analysis only ever listed the unit's own SSIDs.

What

• Scan each interface as its own command (CmdScanIface) with its own 8 s budget, enumerated from a single iw dev call that also feeds the own-BSSID section.
• A slow or stuck radio now only empties its own section instead of taking down every interface's results.
• The monolithic CmdScan literal stays exported, deprecated, for vendored module consumers.
• OpenWrt busybox has no timeout applet, so the isolation lives in the Go runner, not in shell.

Tests

• New scan_periface_test.go: a dead 5 GHz scan no longer starves the 2.4 GHz results (fixture is the exact scan output the reporter pasted, including its double last seen lines), throttle still governs per-interface scans, no-iw units stay quiet.
• scan_throttle_test.go updated to the per-interface commands.
• Agent module: vet clean, full suite green. Server: 48/48 packages with -count=1.

Refs #1338