VPP LB counters, src-ip-sticky, and frontend state aggregation

New feature: per-VIP / per-backend runtime counters
  * New GetVPPLBCounters RPC serving an in-process snapshot refreshed
    by a 5s scrape loop (internal/vpp/lbstats.go). Each cycle pulls
    the LB plugin's four SimpleCounters (next, first, untracked,
    no-server) plus the FIB /net/route/to CombinedCounter for every
    VIP and every backend host prefix via a single DumpStats call.
  * FIB stats-index discovery via ip_route_lookup (internal/vpp/
    fibstats.go); per-worker reduction happens in the collector.
  * Prometheus collector exports vip_packets_total (kind label),
    vip_route_{packets,bytes}_total, and backend_route_{packets,
    bytes}_total. Metrics source interface extended with VIPStats /
    BackendRouteStats; vpp.Client publishes snapshots via
    atomic.Pointer and clears them on disconnect.
  * New 'show vpp lb counters' CLI command. The 'show vpp lbstate'
    and 'sync vpp lbstate' commands are restructured under 'show
    vpp lb {state,counters}' / 'sync vpp lb state' to make room
    for the new verb.

New feature: src-ip-sticky frontends
  * New frontend YAML key 'src-ip-sticky' (bool). Plumbed through
    config.Frontend, desiredVIP, and the lb_add_del_vip_v2 call.
  * Reflected in gRPC FrontendInfo.src_ip_sticky and VPPLBVIP.
    src_ip_sticky, and shown in 'show vpp lb state' output.
  * Scraped back from VPP by parsing 'show lb vips verbose' through
    cli_inband — lb_vip_details does not expose the flag. The same
    scrape also recovers the LB pool index for each VIP, which the
    stats-segment counters are keyed on. This is a documented
    temporary workaround until VPP ships an lb_vip_v2_dump.
  * src_ip_sticky cannot be mutated on a live VIP, so a flipped flag
    triggers a tear-down-and-recreate in reconcileVIP (ASes deleted
    with flush, VIP deleted, then re-added). Flip is logged.

New feature: frontend state aggregation and events
  * New health.FrontendState (unknown/up/down) and FrontendTransition
    types. A frontend is 'up' iff at least one backend has a nonzero
    effective weight, 'unknown' iff no backend has real state yet,
    and 'down' otherwise.
  * Checker tracks per-frontend aggregate state, recomputing after
    each backend transition and emitting a frontend-transition Event
    on change. Reload drops entries for removed frontends.
  * checker.Event gains an optional FrontendTransition pointer;
    backend- vs. frontend-transition events are demultiplexed on
    that field.
  * WatchEvents now sends an initial snapshot of frontend state on
    connect (mirroring the existing backend snapshot), subscribes
    once to the checker stream, and fans out to backend/frontend
    handlers based on the client's filter flags. The proto
    FrontendEvent message grows name + transition fields.
  * New Checker.FrontendState accessor.

Refactor: pure health helpers
  * Moved the priority-failover selector and the (pool idx, active
    pool, state, cfg weight) → (vpp weight, flush) mapping out of
    internal/vpp/lbsync.go into a new internal/health/weights.go so
    the checker can reuse them for frontend-state computation
    without importing internal/vpp.
  * New functions: health.ActivePoolIndex, BackendEffectiveWeight,
    EffectiveWeights, ComputeFrontendState. lbsync.go now calls
    these directly; vpp.EffectiveWeights is a thin wrapper over
    health.EffectiveWeights retained for the gRPC observability
    path. Fully unit-tested in internal/health/weights_test.go.

maglevc polish
  * --color default is now mode-aware: on in the interactive shell,
    off in one-shot mode so piped output is script-safe. Explicit
    --color=true/false still overrides.
  * New stripHostMask helper drops /32 and /128 from VIP display;
    non-host prefixes pass through unchanged.
  * Counter table column order fixed (first before next) and
    packets/bytes columns renamed to fib-packets/fib-bytes to
    clarify they come from the FIB, not the LB plugin.

Docs
  * config-guide: document src-ip-sticky, including the VIP
    recreate-on-change caveat.
  * user-guide, maglevc.1, maglevd.8: updated command tree, new
    counters command, color defaults, and the src-ip-sticky field.

internal/vpp/client.go

@@ -9,6 +9,7 @@ import (
 	"context"
 	"log/slog"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"go.fd.io/govpp/adapter"
@@ -60,6 +61,16 @@ type Client struct {
 	info        Info             // populated on successful connect
 	stateSrc    StateSource      // optional; enables periodic LB sync
 	lastLBConf  *lb.LbConf       // cached last-pushed lb_conf (dedup)
+
+	// lbStatsSnap is the most recent per-VIP stats snapshot captured by
+	// lbStatsLoop. Published as an immutable slice via atomic.Pointer so
+	// Prometheus scrapes (metrics.Collector.Collect) don't take any lock.
+	lbStatsSnap atomic.Pointer[[]metrics.VIPStatEntry]
+
+	// backendRouteSnap is the most recent per-backend FIB stats snapshot
+	// captured by lbStatsLoop. Same atomic-pointer publish pattern as
+	// lbStatsSnap; see logBackendRouteStats in fibstats.go.
+	backendRouteSnap atomic.Pointer[[]metrics.BackendRouteStat]
 }
 
 // SetStateSource attaches a live config + health state source. When set, the
@@ -140,10 +151,11 @@ func (c *Client) Run(ctx context.Context) {
 			}
 		}
 
-		// Start the LB sync loop for as long as the connection is up.
-		// It exits when connCtx is cancelled (on disconnect or shutdown).
+		// Start the LB sync and stats loops for as long as the connection
+		// is up. Both exit when connCtx is cancelled.
 		connCtx, connCancel := context.WithCancel(ctx)
 		go c.lbSyncLoop(connCtx)
+		go c.lbStatsLoop(connCtx)
 
 		// Hold the connection, pinging periodically to detect VPP restarts.
 		c.monitor(ctx)
@@ -217,6 +229,30 @@ func (c *Client) GetInfo() (Info, error) {
 	return c.info, nil
 }
 
+// VIPStats satisfies metrics.VPPSource. It returns the latest snapshot of
+// per-VIP LB stats-segment counters captured by lbStatsLoop. Returns nil
+// until the first scrape completes, or after a disconnect (the pointer is
+// cleared when the connection drops).
+func (c *Client) VIPStats() []metrics.VIPStatEntry {
+	p := c.lbStatsSnap.Load()
+	if p == nil {
+		return nil
+	}
+	return *p
+}
+
+// BackendRouteStats satisfies metrics.VPPSource. It returns the latest
+// snapshot of per-backend FIB combined counters (/net/route/to) captured
+// by lbStatsLoop. Returns nil until the first scrape completes, or after
+// a disconnect.
+func (c *Client) BackendRouteStats() []metrics.BackendRouteStat {
+	p := c.backendRouteSnap.Load()
+	if p == nil {
+		return nil
+	}
+	return *p
+}
+
 // VPPInfo satisfies metrics.VPPSource. It returns a copy of the cached
 // connection info as a metrics-local struct so the metrics package doesn't
 // need to import internal/vpp. Second return is false when VPP is not
@@ -272,6 +308,8 @@ func (c *Client) disconnect() {
 	c.info = Info{}
 	c.lastLBConf = nil // force re-push of lb_conf on reconnect
 	c.mu.Unlock()
+	c.lbStatsSnap.Store(nil)
+	c.backendRouteSnap.Store(nil)
 
 	safeDisconnectAPI(apiConn)
 	safeDisconnectStats(statsConn)