VPP load-balancer dataplane integration: state, sync, and global conf

This commit wires maglevd through to VPP's LB plugin end-to-end, using
locally-generated GoVPP bindings for the newer v2 API messages.

VPP binapi (vendored)
- New package internal/vpp/binapi/ containing lb, lb_types, ip_types, and
  interface_types, generated from a local VPP build (~/src/vpp) via a new
  'make vpp-binapi' target. GoVPP v0.12.0 upstream lacks the v2 messages we
  need (lb_conf_get, lb_add_del_vip_v2, lb_add_del_as_v2, lb_as_v2_dump,
  lb_as_set_weight), so we commit the generated output in-tree.
- All generated files go through our loggedChannel wrapper; every VPP API
  send/receive is recorded at DEBUG via slog (vpp-api-send / vpp-api-recv /
  vpp-api-send-multi / vpp-api-recv-multi) so the full wire-level trail is
  auditable. NewAPIChannel is unexported — callers must use c.apiChannel().

Read path: GetLBState{All,VIP}
- GetLBStateAll returns a full snapshot (global conf + every VIP with its
  attached application servers).
- GetLBStateVIP looks up a single VIP by (prefix, protocol, port) and
  returns (nil, nil) when the VIP doesn't exist in VPP. This is the
  efficient path for targeted updates on a busy LB.
- Helpers factored out: getLBConf, dumpAllVIPs, dumpASesForVIP, lookupVIP,
  vipFromDetails.

Write path: SyncLBState{All,VIP}
- SyncLBStateAll reconciles every configured frontend with VPP: creates
  missing VIPs, removes stale ones (with AS flush), and reconciles AS
  membership and weights within VIPs that exist on both sides.
- SyncLBStateVIP targets a single frontend by name. Never removes VIPs.
  Returns ErrFrontendNotFound (wrapped with the name) when the frontend
  isn't in config, so callers can use errors.Is.
- Shared reconcileVIP helper does the per-VIP AS diff; removeVIP is used
  only by the full-sync pass.
- LbAddDelVipV2 requests always set NewFlowsTableLength=1024. The .api
  default=1024 annotation is only applied by VAT/CLI parsers, not wire-
  level marshalling — sending 0 caused VPP to vec_validate with mask
  0xFFFFFFFF and OOM-panic.
- Pool semantics: backends in the primary (first) pool of a frontend get
  their configured weight; backends in secondary pools get weight 0. All
  backends are installed so higher layers can flip weights on failover
  without add/remove churn.
- Every individual change emits a DEBUG slog (vpp-lbsync-vip-add/del,
  vpp-lbsync-as-add/del, vpp-lbsync-as-weight). Start/done INFO logs
  carry a scope=all|vip label plus aggregate counts.

Global conf push: SetLBConf
- New SetLBConf(cfg) sends lb_conf with ipv4-src, ipv6-src, sticky-buckets,
  and flow-timeout. Called automatically on VPP (re)connect and after
  every config reload (via doReloadConfig). Results are cached on the
  Client so redundant pushes are silently skipped — only actual changes
  produce a vpp-lb-conf-set INFO log line.

Periodic drift reconciliation
- vpp.Client.lbSyncLoop runs in a goroutine tied to each VPP connection's
  lifetime. Its first tick is immediate (startup and post-reconnect
  sync quickly); subsequent ticks fire every vpp.lb.sync-interval from
  config (default 30s). Purpose: catch drift if something/someone
  modifies VPP state by hand. The loop uses a ConfigSource interface
  (satisfied by checker.Checker via its new Config() accessor) to avoid
  an import cycle with the checker package.

Config schema additions (maglev.vpp.lb)
- sync-interval: positive Go duration, default 30s.
- ipv4-src-address: REQUIRED. Used as the outer source for GRE4 encap
  to application servers. Missing this is a hard semantic error —
  maglevd --check exits 2 and the daemon refuses to start. VPP GRE
  needs a source address and every VIP we program uses GRE, so there
  is no meaningful config without it.
- ipv6-src-address: REQUIRED. Same treatment as ipv4-src-address.
- sticky-buckets-per-core: default 65536, must be a power of 2.
- flow-timeout: default 40s, must be a whole number of seconds in [1s, 120s].
- VPP validation runs at the end of convert() so structural errors in
  healthchecks/backends/frontends surface first — operators fix those,
  then get the VPP-specific requirements.

gRPC API
- New GetVPPLBState RPC returning VPPLBState: global conf + VIPs with
  ASes. Mirrors the read-path but strips fields irrelevant to our
  GRE-only deployment (srv_type, dscp, target_port).
- New SyncVPPLBState RPC with optional frontend_name. Unset → full sync
  (may remove stale VIPs). Set → single-VIP sync (never removes).
  Returns codes.NotFound for unknown frontends, codes.Unavailable when
  VPP integration is disabled or disconnected.

maglevc (CLI)
- New 'show vpp lbstate' command displaying the LB plugin state. VPP-only
  fields the dataplane irrelevant to GRE are suppressed. Per-AS lines use
  a key-value format ("address X  weight Y  flow-table-buckets Z")
  instead of a tabwriter column, which avoids the ANSI-color alignment
  issue we hit with mixed label/data rows.
- New 'sync vpp lbstate [<name>]' command. Without a name, triggers a
  full reconciliation; with a name, targets one frontend.
- Previous 'show vpp lb' renamed to 'show vpp lbstate' for consistency
  with the new sync command.

Test fixtures
- validConfig and all ad-hoc config_test.go fixtures that reach the end
  of convert() now include the two required vpp.lb src addresses.
- tests/01-maglevd/maglevd-lab/maglev.yaml gains a vpp.lb section so the
  robot integration tests can still load the config.
- cmd/maglevc/tree_test.go gains expected paths for the new commands.

Docs
- config-guide.md: new 'vpp' section in the basic structure, detailed
  vpp.lb field reference, noting ipv4/ipv6 src addresses as REQUIRED
  (hard error) with no defaults; example config updated.
- user-guide.md: documented 'show vpp info', 'show vpp lbstate',
  'sync vpp lbstate [<name>]', new --vpp-api-addr and --vpp-stats-addr
  flags, the vpp-lb-conf-set log line, and corrected the pause/resume
  description to reflect that pause cancels the probe goroutine.
- debian/maglev.yaml: example config gains a vpp.lb block with src
  addresses and commented optional overrides.

internal/vpp/apilog.go

@@ -0,0 +1,122 @@
+// Copyright (c) 2026, Pim van Pelt <pim@ipng.ch>
+
+package vpp
+
+import (
+	"fmt"
+	"log/slog"
+
+	"go.fd.io/govpp/api"
+)
+
+// loggedChannel wraps an api.Channel so that every VPP request/reply is
+// recorded via slog at DEBUG level. All code in this package MUST send VPP
+// messages through a loggedChannel (via Client.apiChannel) so we have a
+// complete audit trail of what was sent to the dataplane.
+type loggedChannel struct {
+	ch api.Channel
+}
+
+// apiChannel opens a new API channel wrapped in logging. This is the only
+// approved way to talk to VPP; do not call conn.NewAPIChannel directly.
+func (c *Client) apiChannel() (*loggedChannel, error) {
+	c.mu.Lock()
+	conn := c.apiConn
+	c.mu.Unlock()
+	if conn == nil {
+		return nil, errNotConnected
+	}
+	ch, err := conn.NewAPIChannel()
+	if err != nil {
+		return nil, err
+	}
+	return &loggedChannel{ch: ch}, nil
+}
+
+// Close closes the underlying channel.
+func (lc *loggedChannel) Close() { lc.ch.Close() }
+
+// SendRequest logs the outgoing message and returns a wrapped request context.
+func (lc *loggedChannel) SendRequest(msg api.Message) *loggedRequestCtx {
+	slog.Debug("vpp-api-send",
+		"msg", msg.GetMessageName(),
+		"crc", msg.GetCrcString(),
+		"payload", fmt.Sprintf("%+v", msg),
+	)
+	return &loggedRequestCtx{
+		ctx:  lc.ch.SendRequest(msg),
+		name: msg.GetMessageName(),
+	}
+}
+
+// SendMultiRequest logs the outgoing message and returns a wrapped multi-request context.
+func (lc *loggedChannel) SendMultiRequest(msg api.Message) *loggedMultiRequestCtx {
+	slog.Debug("vpp-api-send-multi",
+		"msg", msg.GetMessageName(),
+		"crc", msg.GetCrcString(),
+		"payload", fmt.Sprintf("%+v", msg),
+	)
+	return &loggedMultiRequestCtx{
+		ctx:  lc.ch.SendMultiRequest(msg),
+		name: msg.GetMessageName(),
+	}
+}
+
+// loggedRequestCtx wraps api.RequestCtx and logs the reply on ReceiveReply.
+type loggedRequestCtx struct {
+	ctx  api.RequestCtx
+	name string
+}
+
+func (r *loggedRequestCtx) ReceiveReply(msg api.Message) error {
+	err := r.ctx.ReceiveReply(msg)
+	if err != nil {
+		slog.Debug("vpp-api-recv",
+			"req", r.name,
+			"reply", msg.GetMessageName(),
+			"err", err,
+		)
+		return err
+	}
+	slog.Debug("vpp-api-recv",
+		"req", r.name,
+		"reply", msg.GetMessageName(),
+		"payload", fmt.Sprintf("%+v", msg),
+	)
+	return nil
+}
+
+// loggedMultiRequestCtx wraps api.MultiRequestCtx and logs each reply.
+type loggedMultiRequestCtx struct {
+	ctx  api.MultiRequestCtx
+	name string
+	seq  int
+}
+
+func (r *loggedMultiRequestCtx) ReceiveReply(msg api.Message) (bool, error) {
+	stop, err := r.ctx.ReceiveReply(msg)
+	if err != nil {
+		slog.Debug("vpp-api-recv-multi",
+			"req", r.name,
+			"reply", msg.GetMessageName(),
+			"seq", r.seq,
+			"err", err,
+		)
+		return stop, err
+	}
+	if stop {
+		slog.Debug("vpp-api-recv-multi-done",
+			"req", r.name,
+			"count", r.seq,
+		)
+		return stop, nil
+	}
+	slog.Debug("vpp-api-recv-multi",
+		"req", r.name,
+		"reply", msg.GetMessageName(),
+		"seq", r.seq,
+		"payload", fmt.Sprintf("%+v", msg),
+	)
+	r.seq++
+	return stop, nil
+}