[sidecar/pipeline] P4 cleanup and a return to 18 stages

[zeeshanlakhani]

P4/sidecar pipeline changes:

• Unify route_result_t struct for Router4/Router6 to prevent PHV liverange divergence
• Unify nexthop: replace separate nexthop_ipv4/nexthop_ipv6 with single nexthop (ipv6_addr_t) plus nexthop_is_v6 flag
• Add TTL compound key (idx, route_ttl_is_1) to route tables for TTL offload
  • This includes Rust updates and a larger routing table for the compound key
• Add @pa_no_init pragmas for metadata fields to guard against compiler init bugs
• Add @pa_container_type("normal") pragmas for all ingress booleans and wider metadata fields to prevent mocha container corruption (whole-container writes clobbering neighboring fields)
• Extend @pa_container_type to egress bridge header fields and drop_reason
• Zero sc_pad at every sc_code write site (was uninitialized before)
• Bridge header now includes is_mcast_routed for CPU copy detection

EgressFilter:

• Moves EgressFilter from ingress to egress pipeline to free one ingress stage for NatEgressFilter
• Adds nat_egress_hit to bridge header to carry NAT state across TM boundary
• Control Plane: updates table path and match key (egress_port instead of ucast_egress_port)

Egress MacRewrite:

• Creates separate unicast_mac_rewrite and mcast_mac_rewrite instances of MacRewrite
• Unicast instance: rewrites src_mac only (dst_mac already correct from ARP/NDP)
• Multicast instance: derives dst_mac from group address per RFC 1112/2464

Multicast Egress changes:

• Checks egress_rid != 0 first to identify PRE-replicated packets
• Validates is_mcast_routed for CPU copies (egress_rid == 0 but routed to multicast)
• Drops any CPU copies with DROP_MULTICAST_CPU_COPY reason

Counters:

• Moves Unicast, MulticastLL, EgressDropPort, EgressDropReason counters from multicast to base
• Adds a Forwarded counter (for every packet copy that egresses the pipeline)
• Removes Egress counter (from ingress pipe, replaced by per-port counters)
• Removes MulticastDrop from MULTICAST_COUNTERS, as it's covered in general drop w/ reason
• Handles link-local multicast counting in both MULTICAST and non-MULTICAST paths

Build:

• Configures TOFINO_STAGES where there's 15 for base, 18 for multicast (the win!)

[zeeshanlakhani]

Decided to target main as we're gated throughout for mcast's 18 stages.