lacp: implement IEEE 802.3ad link aggregation

Dynamic link aggregation on top of the static LAGG, for RTL8372/8373:
receive, selection and mux state machines (coupled mux, one aggregator
per group), with "lacp show" to look inside. Each of the four trunk
groups runs its own aggregator on the candidate ports assigned with
"lag <n> lacp <ports>"; the engine starts with the first such group and
stops with the last.

LACPDUs reach the CPU through the reserved-multicast trap, so delivery
needs no L2 entry and nothing written per VLAN or per PVID. The trap
acts on the address before the VLAN lookup and before the ingress
filters, so a LACPDU also arrives on a port that admits tagged frames
only and on a port STP holds in blocking, neither of which a forwarded
one reaches. A trapped frame carries no VLAN tag,
so lacp_in() takes the offset of the PDU body from TRAP_RX_BODY(). The
action is set with trap_rma_set(), which leaves the flags sharing that
register alone, and "lacp off" puts the slow-protocols drop default
back. doc/lacp.md and doc/link_aggregation.md record what was measured,
including why the trap looked like a dead end at first.

The web UI gets a Static/LACP mode per group on the LAG page with live
status, and the startup-config validator knows the per-group syntax. A
host-side sandbox for the state machines runs in the test harness
together with the LAG hash check. It drives both forms of an incoming
LACPDU, so the body offset of a trapped one is covered as well.

The L2 table page iterated with the "next unicast" read method, so
static multicast entries such as an IGMP group were invisible. It now
iterates with "next address", skips L3 entries (their layout carries IP
addresses where a MAC is expected), and reports the lowest member of a
multicast entry's port mask, so such an entry shows up as a static
entry on that port.

A port taken out of LACP forgets its partner, so "lacp show" after
"lacp off" no longer reports one. Applying the same candidate set to a
group again, as the LAG page does on every Apply, leaves the trunk
alone instead of tearing it down for a second or two. lacp_in() checks
the frame length and ethertype before reading a LACPDU, so a runt or a
foreign frame on the slow-protocols address cannot leave stale buffer
bytes as a partner. The sandbox covers these three in T20-T22. On the
LAG page, LACP mode with no ports checked sends "lag N lacp off", and
the startup-config validator wants at least one port after "lacp".

test/env_tables.c

@@ -16,6 +16,7 @@
 #include "syslog.h"
 #include "rtl837x_phy.h"
 #include "support.h"
+#include "rtl837x_lacp.h"
 
 /* ---- console: everything lands in out_buf (support.c) ---- */
 void print_byte(uint8_t v)
@@ -124,3 +125,18 @@ uint8_t  stp_prio = 0x80, stp_root_port = 0xff, stp_rstp = 1, stp_txhold = 6;
 uint8_t sfp_buf[16];
 bool    sfp_read_block(uint8_t slot, uint8_t reg, uint8_t len) { (void)slot; (void)reg; (void)len; return false; }
 void    print_phys_port(uint8_t port) { itoa_short(machine.log_to_phys_port[port]); }
+
+uint8_t  lacpEnabled;
+uint8_t  lacp_actor_state[10];
+uint8_t  lacp_partner_state[10];
+uint8_t  lacp_rx_state[10];
+uint8_t  lacp_partner_sys[10][6];
+uint16_t lacp_rx_count[10];
+uint8_t  lacp_port_lag[10] = {
+	LACP_LAG_NONE, LACP_LAG_NONE, LACP_LAG_NONE, LACP_LAG_NONE, LACP_LAG_NONE,
+	LACP_LAG_NONE, LACP_LAG_NONE, LACP_LAG_NONE, LACP_LAG_NONE, LACP_LAG_NONE,
+};
+uint16_t lacp_lag_ports[LACP_NUM_LAGS];
+uint8_t  lacp_agg_sys[LACP_NUM_LAGS][6];
+uint8_t  lacp_agg_valid[LACP_NUM_LAGS];
+uint16_t lacp_members_last[LACP_NUM_LAGS];