config_parse_route_section: most ltypes are validated against the route-Type enum grammar

[SJrX]

Problem

ConfigParseRouteSectionOptionValue implements the route Type enum grammar
(unicast, blackhole, unreachable, …), but in AiGenerated it is registered for
nine config_parse_route_section ltypes — only one of which (ROUTE_TYPE) actually
takes that enum. The rest take numbers, booleans, or different enums, so their valid
values are mis-flagged as invalid.

Discovered while adding Gateway coverage (#475); the experimental key-marker (#474) makes
these easy to spot.

Mis-registered ltypes (correctness bug)

All of these currently resolve to the route-Type word list, which is wrong:

ltype	.network key(s)	actually accepts	example wrongly flagged
ROUTE_PRIORITY	Route.Metric	unsigned integer	Metric=100
ROUTE_PROTOCOL	Route.Protocol	kernel/boot/static/dhcp or 0–255	Protocol=static
ROUTE_PREFERENCE	Route.IPv6Preference	low/medium/high	IPv6Preference=high
ROUTE_NEXTHOP	Route.NextHop	nexthop id (unsigned int)	NextHop=1
ROUTE_METRIC_HOPLIMIT	Route.HopLimit	unsigned integer	HopLimit=64
ROUTE_METRIC_INITRWND	Route.InitialAdvertisedReceiveWindow	unsigned integer	InitialAdvertisedReceiveWindow=10
ROUTE_METRIC_QUICKACK	Route.QuickAck	boolean	QuickAck=true
ROUTE_METRIC_FASTOPEN_NO_COOKIE	Route.FastOpenNoCookie	boolean	FastOpenNoCookie=yes

Only ROUTE_TYPE (Route.Type) is correctly served by the enum grammar.

Also: unregistered ltypes (coverage gaps — no validation today)

These config_parse_route_section ltypes appear in the networkd gperf but have no entry
in AiGenerated, so their values aren't validated at all (same situation Gateway was in
before #475):

• ROUTE_DESTINATION — Route.Destination, Route.Source (address/prefix)
• ROUTE_SCOPE — Route.Scope (global/site/link/host/nowhere or number)
• ROUTE_PREFERRED_SOURCE — Route.PreferredSource (address)
• ROUTE_TABLE — Route.Table (default/main/local or number)
• ROUTE_GATEWAY_ONLINK — Route.GatewayOnLink / Route.GatewayOnlink (boolean)
• ROUTE_MULTIPATH — Route.MultiPathRoute
• ROUTE_METRIC_MTU — Route.MTUBytes (size)
• ROUTE_METRIC_ADVMSS — Route.TCPAdvertisedMaximumSegmentSize (size)
• ROUTE_METRIC_INITCWND — Route.InitialCongestionWindow (unsigned int)
• ROUTE_METRIC_RTO_MIN — Route.TCPRetransmissionTimeoutSec (time)
• ROUTE_METRIC_CC_ALGO — Route.TCPCongestionControlAlgorithm (string)

Suggested fix

Give each ltype its own correct grammar (as done for the gateway ltypes in #475) rather
than sharing the Type-enum validator. The mis-registrations are the priority (they cause
false errors on valid configs); the unregistered ltypes are latent gaps. The key-marker
behind the experimental flag is handy for verifying each one lights up once covered.

Refs #467 #475