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.

docs/config-guide.md

@@ -288,6 +288,15 @@ ordered list of backend pools. The gRPC API exposes frontends by name.
     cascades across further tiers. See [healthchecks.md](healthchecks.md#pool-failover)
     for the full failover semantics. All backends across all pools in a frontend must
     have addresses of the same address family (all IPv4 or all IPv6).
+*   ***src-ip-sticky***: Boolean, default `false`. When `true`, the VPP load-balancer
+    programs this VIP with source-IP-based stickiness — all flows from the same client
+    source IP hash to the same backend (subject to the Maglev consistent-hash bucket
+    assignment). Use this for protocols that require session affinity at the L3 level,
+    or when clients open many short flows that should land on one backend. Changing this
+    field in a running config and reloading causes maglevd to tear down the VIP (all
+    application servers are deleted with flush, then the VIP itself is deleted) and
+    recreate it with the new value; VPP has no API to mutate `src_ip_sticky` on an
+    existing VIP, and existing flow state cannot be preserved across the flip.
 
 Each pool has:
 
@@ -324,6 +333,7 @@ frontends:
     address: 2001:db8::1
     protocol: tcp
     port: 993
+    src-ip-sticky: true
     pools:
       - name: primary
         backends: