Add optional steering_ssid allow-list to scope DAWN per SSID

[lePaulo]

Motivation

There is currently no way to run DAWN on some SSIDs but not others. DAWN
subscribes to every hostapd BSS it discovers under hostapd_dir, and there
is no UCI option (nor an OpenWrt wireless option) to scope it. On APs that host
a mix of networks — e.g. a steered main/roaming SSID alongside a static IoT or
guest SSID that should be left completely alone — this is an all-or-nothing
choice today.

What this adds

An opt-in allow-list in the config hostapd section:

config hostapd
    option hostapd_dir '/var/run/hostapd'
    list steering_ssid 'MyMainNetwork'
    list steering_ssid 'MyRoamingNetwork'

• With no steering_ssid entries the list is empty and behaviour is
  unchanged — DAWN manages every BSS, exactly as before. Fully backwards
  compatible.
• With one or more entries, only BSSes whose SSID is listed are managed.

Implementation

Filtering happens at interface discovery in subscribe_to_new_interfaces().
A BSS whose SSID is not allowed is simply never subscribed, so DAWN neither
steers/kicks its clients nor shares their measurements with peer APs — the
interface is invisible to DAWN rather than partially handled.

• The SSID is read with the existing get_ssid(ifname, ...) (iwinfo), which
  works at discovery time, so no extra subscription is needed to make the
  decision.
• Fail-open: if the SSID cannot be read, the interface is managed as
  before, so the feature can never accidentally disable steering.
• The check only runs for not-yet-subscribed interfaces, so steady-state
  overhead is nil; an excluded BSS is re-evaluated on each rescan, so it can be
  picked up later if its SSID changes.
• The allow-list is loaded once via a new uci_get_dawn_steering_ssids()
  (mirroring uci_get_dawn_hostapd_dir()); dawn_ssid_is_managed() does the
  membership test.

Caveat

Like hostapd_dir, the config hostapd section is not covered by
ubus call dawn reload_config, so changes to steering_ssid take effect on
service restart. This is noted in CONFIGURE.md.

Docs

CONFIGURE.md (regenerated CONFIG-H row + prose) and the example
dawn-config are updated.

Testing

• Cross-compiled for mediatek/filogic (aarch64_cortex-a53) against the
  OpenWrt 25.12.4 SDK — clean build under DAWN's -Wall -Werror --std=gnu99 -Wmissing-declarations.
• Installed and runtime-tested on MT7981 hardware (OpenWrt 25.12.4): an
  excluded SSID's clients are no longer steered/kicked, while the listed SSID
  continues to be managed normally. Log confirms the skip:
  [SUBSCRIBING] Skipping <iface>: SSID '<other>' not in steering_ssid list.

🤖 Generated with Claude Code

[PolynomialDivision]

Fine for me. But if we dont subscribe to each "ssid", we maybe loose some information how many clients are in total on a radio and similar. However, for now I dont think this is important.

[lePaulo]

Thanks! Agreed it's not critical for now — and good point to flag.

Small clarification on the metrics: channel utilization actually survives, since get_channel_utilization() reads the channel-wide survey (busy_time/active_time), which already includes the airtime of the other BSSes on the radio — so an unsubscribed SSID's load is still reflected there.

The one that does undercount is station_count, and only when an excluded SSID shares a radio with a managed one and use_station_count is enabled. If that ever matters, a natural follow-up would be a "subscribe for metrics but never steer" mode — happy to look at it in a later PR if you'd want that.

Since this is opt-in (empty steering_ssid = manage everything, as today), it doesn't change any existing deployment.

Cheers!

[lePaulo]

@PolynomialDivision - thanks for merging. Can you close the following issue then please: #239