vrrp-wan: size GRACE from the measured hostile failover, not the theory

With the matrix actually setting session-control, T4 timed a destroyed
master's takeover at:

    replace    26s
    deny      148s   <-- sizing case
    disable    21s

`deny` is the case GRACE exists for: the AC refuses the survivor until its own
dead-peer timer frees the dead session. The session poller caught it happening
-- the destroyed router's session stayed in the table while the survivor's
dials appeared and were rejected, twice, before one took at 148s.

148s is well past the lcp-echo-interval(30) x failure(3) = 90s budget that 180
was sized against, leaving 32s of margin. GRACE=300 is ~2x the worst observed.
Treat 148s as a floor, not a worst case: these are idle 2-vCPU VMs, and the AC
shares an OVS bridge with the routers, so `virsh destroy` removes the port and
accel-ppp sees the peer vanish. A real BRAS over DSL never learns our router
died and waits out longer timers of its own.

The cost is stated in the conf: GRACE is also how long an alive-but-unroutable
master holds every VIP before yielding. bond0.53 covers most of that -- a DHCP
lease satisfies the check in seconds -- so GRACE only dominates when PPPoE is
the last path. Kept the health check's fallback in step, since keepalived runs
it with no environment and that number decides mastership if the conf is ever
missing.

check_invariant no longer fails blind on the AC's session count. That count is
only a proxy for "two of our routers dialled", and only while the AC enforces
single-session; under `disable` it does not, so a destroyed router's session
lingers and the count reads 2 with exactly one live router dialled. The real
invariant -- at most one router holds pppoe0 -- is now the failing one, and the
stale session is reported as a WARN rather than silenced, because it still
occupies the slot at a real ISP and is precisely what made `deny` take 148s.

migration/vrrp-wan-health

@@ -40,7 +40,14 @@ VIP="${VRRP_WAN_VIP:-192.168.1.1}"
 # the boundary: the new master fails its own check, sheds the VIPs, and the peer
 # -- in the same position -- does likewise. Both end in FAULT, which is worse
 # than the outage this check exists to prevent.
-GRACE="${GRACE:-180}"
+#
+# The fallback tracks vrrp-wan.conf, where the reasoning and the measurements
+# live. Short version: labsim T4 timed a destroyed master's takeover at 26s
+# (replace), 148s (deny) and 21s (disable); `deny` is the sizing case and 180
+# left only 32s over it. Keep the two in step -- keepalived runs this script
+# with no environment, so if vrrp-wan.conf is ever missing THIS number is the
+# one that decides mastership.
+GRACE="${GRACE:-300}"
 
 # A deliberate hand-over lever.
 #