feat(openwrt): open !FIPS access SSID layer (#126)

Access layer for phones and laptops to reach FIPS routers on OpenWrt,
stacked on the 802.11s mesh backhaul from #123. Squashed from five
commits by Arjen (Origami74); their original messages follow.

* feat(openwrt): open !FIPS access SSID — fips-ap-setup helper, default transport binding, how-to

  Client access layer for phones and laptops: every FIPS router
  broadcasts the same open SSID ('!FIPS' — the leading '!' sorts it to
  the top of alphabetically ordered network pickers), forming one
  standard ESS. Clients save it once and roam between all FIPS routers
  natively, with FIPS's Noise IK handshake as the only security layer:

  - fips-ap-setup: opt-in UCI helper that creates the 'fips-ap0' open AP
    (encryption none — security type must be uniform across routers or
    clients treat the ESS as different saved networks), an isolated
    network with a static ULA /64, RA-only odhcpd addressing (stateless
    SLAAC, no DHCP — the minimum that satisfies Android's provisioning
    check; no internet by design, so phones keep cellular as default
    route), and a locked-down fips_ap firewall zone (no path to br-lan
    or the WAN; only ICMPv6, mDNS, and the FIPS transports reachable).
    'remove' subcommand undoes it. Radio setup stays opt-in; a package
    must not commandeer radios on install.
  - fips.yaml: ship 'ap0'/'ap1' Ethernet-transport entries commented out
    (matching the 802.11s mesh backhaul) so a stock install that never
    creates fips-ap* logs no per-boot "interface missing" bind warning;
    fips-ap-setup uncomments the matching block when it creates the
    interface and re-comments it on remove.
  - Regression test: extend shipped_openwrt_config_parses to assert the
    ap0/ap1 entries ship commented out and still parse once uncommented,
    alongside mesh0/mesh1.
  - Packaging: install the helper in ipk/apk/buildroot (three synced
    copies), extend CI structural checks and shellcheck targets.
  - docs/how-to/set-up-open-access-ssid.md: full guide, including the
    one-time 'no internet, stay connected' acceptance (stored per SSID,
    covers every FIPS router) and the security-type-uniformity
    constraint.

* docs(openwrt): correct open-SSID/mesh security framing — peering is open by design

  The fips-ap-setup/fips-mesh-setup comments and both how-tos claimed a
  stranger "cannot pass the FIPS handshake" / "their frames die at the
  handshake" / the handshake surface "drops them". That is wrong: FIPS
  peer admission is open. An inbound handshake from any net-new identity
  is promoted (node::handlers::handshake::promote_connection), gated only
  by the daemon's max-peers cap — there is no allowlist, no PSK, and the
  AuthChallenge path is not wired to admission. The Noise IK handshake
  provides authentication (no impersonation of another identity, no MITM),
  not authorization.

  Restate the model accurately in all five places: a stranger on the open
  SSID (or the open mesh) can associate AND form a FIPS peer link — that
  is the point of open access. Containment is the isolated fips_ap zone
  (no path to br-lan or the WAN) plus the max-peers cap, not the
  handshake. Clarify that AP client isolation is an L2 control only: a
  peered stranger is an overlay peer like any other, so the FIPS overlay,
  not L2, is the trust boundary between clients.

* feat(openwrt): serve DHCPv4 on the access SSID from a fixed roamable subnet

  RA-only addressing satisfied Android's provisioning check but left
  anything expecting IPv4 with a self-assigned address and a "no IP"
  complaint. dnsmasq now leases out of 10.21.<N>.0/24 (N = radio index;
  prefix echoes FIPS port 2121), deliberately identical on every router:
  a roaming client keeps its lease across the ESS, and dnsmasq's
  authoritative mode — the OpenWrt default, pinned by the helper — ACKs
  the renew a foreign router never issued. Lease collisions across
  routers surface as a NAK on renew and the client re-DHCPs.

  The dhcp section's 'dhcpv4 server' is read by both dnsmasq (default
  images) and odhcpd (only with maindhcp), so either arrangement serves.
  A DHCPv4/udp-67 accept rule joins the fips_ap zone; DHCPv6 stays off,
  the ULA RA stays as-is, and nothing depends on an upstream. The zone
  remains isolated — no forwardings, no internet.

* feat(openwrt): enable mDNS rendezvous from fips-ap-setup

  Phone FIPS apps cannot open raw-Ethernet sockets, so DNS-SD is how they
  find the router's daemon — but node.rendezvous.lan defaults to off. Ship
  the lan block commented in fips.yaml (consistent with the apN transport
  entries) and have fips-ap-setup uncomment it when creating the access
  SSID. The awk match is scoped to node.rendezvous because
  transports.ethernet carries a 'lan' entry at the same indent. The switch
  is daemon-wide, so 'remove' deliberately leaves it on rather than guess
  whether other transports rely on it.

* fix(openwrt): bind UDP dual-stack [::]:2121 so access-SSID clients reach it

  The shipped router config bound the UDP transport "0.0.0.0:2121" (IPv4
  wildcard) while the mDNS LAN advert announces every interface address,
  including the router's IPv6 link-local — which phones on the !FIPS
  access SSID rightly prefer (their cellular default route swallows v4,
  and fd00::/8 is captured by the Myco mesh TUN). Result: the client's
  Noise msg1 arrives on an unbound v6 port and is silently lost; the
  handshake resends and times out.

  Symptom chain (observed on-device): mDNS resolve OK, platform push OK,
  "Sent Noise handshake message 1" to [fe80::…%N]:2121, four resends, no
  reply, 30 s stale-timeout.

  OpenWrt is Linux (bindv6only=0), so "[::]" accepts IPv4 via v4-mapped
  addresses too — nothing is lost. packaging/common is deliberately left
  on "0.0.0.0" for now: Windows defaults IPV6_V6ONLY=1, where "[::]"
  would drop v4 instead.

.github/workflows/package-openwrt.yml

@@ -286,6 +286,7 @@ jobs:
             "$FILES_DIR/etc/hotplug.d/net/99-fips"
             "$FILES_DIR/etc/uci-defaults/90-fips-setup"
             "$FILES_DIR/usr/bin/fips-mesh-setup"
+            "$FILES_DIR/usr/bin/fips-ap-setup"
           )
           fail=0
           for f in "${TARGETS[@]}"; do
@@ -406,6 +407,7 @@ jobs:
             ./usr/bin/fipstop
             ./usr/bin/fips-gateway
             ./usr/bin/fips-mesh-setup
+            ./usr/bin/fips-ap-setup
             ./etc/init.d/fips
             ./etc/init.d/fips-gateway
             ./etc/fips/fips.yaml
@@ -719,7 +721,7 @@ jobs:
 
           for path in \
             usr/bin/fips usr/bin/fipsctl usr/bin/fipstop usr/bin/fips-gateway \
-            usr/bin/fips-mesh-setup \
+            usr/bin/fips-mesh-setup usr/bin/fips-ap-setup \
             etc/init.d/fips etc/init.d/fips-gateway \
             etc/fips/fips.yaml etc/fips/firewall.sh etc/dnsmasq.d/fips.conf \
             etc/sysctl.d/fips-gateway.conf etc/sysctl.d/fips-bridge.conf \