New maglev-frontend component; promote LB sync events to INFO

Introduces maglev-frontend, a responsive, real-time web dashboard for one
or more running maglevd instances. Source lives at cmd/frontend/; the
built binary is maglev-frontend. It is a single Go process with the
SolidJS SPA embedded via //go:embed — no runtime file dependencies.

Architecture
 - One persistent gRPC connection per configured maglevd (-server A,B,C).
   Each connection runs three background loops: a WatchEvents stream
   subscribed at log_level=debug for live events, a 30s refresh loop as
   a safety net for drift, and a 5s health loop that surfaces connection
   drops quickly.
 - In-process pub/sub broker with a 30s / 2000-event replay ring using
   <epoch>-<seq> monotonic IDs. Short browser reconnects (nginx idle,
   wifi flap, laptop wake) silently replay buffered events via the
   EventSource Last-Event-ID header; longer outages or frontend restarts
   fall through to a "resync" event that triggers a full state refetch.
 - HTTP surface: /view/ (SPA), /view/api/state, /view/api/state/{name},
   /view/api/maglevds, /view/api/version, /view/api/events (SSE),
   /healthz, and an /admin/* placeholder returning 501 for a future
   basic-auth mutation surface.
 - SSE handler follows the full operational checklist: retry hint, 15s
   : ping heartbeat, Flush after every write, r.Context().Done() teardown,
   X-Accel-Buffering: no, and no gzip.

SolidJS SPA (cmd/frontend/web/, Vite + TypeScript)
 - solid-js/store for a reactive per-maglevd state tree; reducers apply
   backend transitions, maglevd-status flips, and resync refetches.
 - Scope selector tabs for multi-maglevd support, per-maglevd frontend
   cards with pool tables showing state, configured weight, effective
   weight, and last-transition age.
 - ProbeHeartbeat component turns a middle-dot into ❤️ on probe-start and
   back on probe-done, driven by real log events; fixed-size wrapper so
   the emoji swap doesn't jiggle the row.
 - Flash wrapper animates any primitive on change (1s yellow fade via
   Web Animations API, skipped on first mount). Wired into the state
   badge, configured weight, and effective weight columns.
 - DebugPanel: chronological rolling event tail with tail-style auto-
   scroll, pause/resume, and scope/firehose filter. Syntactic highlight
   for vpp-lb-sync-* events with fixed-order attribute formatting.
 - Live effective_weight updates: vpp-lb-sync-as-added/removed/weight-
   updated log events are routed through a reducer that walks the
   snapshot's pool rows and sets effective_weight on every match
   without waiting for the 30s refresh.
 - Header shows build version + commit with build date in a tooltip,
   fetched once from /view/api/version on mount.
 - Prettier wired in as the web-side fixstyle; make fixstyle now tidies
   both Go and web in one shot via a new fixstyle-web target.

Per-mutation VPP LB sync logging
 - Promotes the addVIP/delVIP/addAS/delAS/setASWeight helpers from
   slog.Debug to slog.Info and renames them from vpp-lbsync-* to
   vpp-lb-sync-{vip-added,vip-removed,as-added,as-removed,as-weight-
   updated}. Matching rename for vpp-lb-sync-start / -done / -error /
   -vip-recreate. The Prometheus metric name (maglev_vpp_lbsync_total)
   is left alone to preserve dashboards.
 - setASWeight now takes the prior weight so the event can emit
   from=X to=Y and the UI can show the delta.
 - The vip field in every event is the bare address (no /32 or /128
   mask), matching the CLI output style.
 - Any listener on the gRPC WatchEvents stream — CLI watch events or
   maglev-frontend — now sees every VIP/AS dataplane change in real
   time without needing to raise the log level.

Build and tooling
 - Makefile: maglev-frontend added to BINARIES; build / build-amd64 /
   build-arm64 emit the binary alongside maglevd and maglevc. A new
   maglev-frontend-web target rebuilds the SolidJS bundle via npm.
 - web/dist/ is tracked so a bare `go build` keeps working for Go-only
   contributors and CI.
 - .gitignore skips cmd/frontend/web/node_modules/.

Stability fixes
 - maglevd's WatchEvents synthetic replay events (from==to, at_unix_ns=0)
   were corrupting the frontend's LastTransition cache with at=0,
   rendering as "20555d ago" in the browser. Client now skips synthetic
   events: the cache comes from refreshAll and doesn't need them.
 - Frontends, Backends, and HealthChecks are now served in the order
   returned by the corresponding List* RPC instead of Go map iteration
   order, so reloads and refreshes keep the SPA stable.

cmd/frontend/web/src/api/rest.ts

@@ -0,0 +1,23 @@
+import type { MaglevdInfo, StateSnapshot, VersionInfo } from "../types";
+
+async function getJSON<T>(path: string): Promise<T> {
+  const r = await fetch(path, { credentials: "same-origin" });
+  if (!r.ok) throw new Error(`${path}: ${r.status} ${r.statusText}`);
+  return (await r.json()) as T;
+}
+
+export function listMaglevds(): Promise<MaglevdInfo[]> {
+  return getJSON<MaglevdInfo[]>("/view/api/maglevds");
+}
+
+export function fetchAllState(): Promise<StateSnapshot[]> {
+  return getJSON<StateSnapshot[]>("/view/api/state");
+}
+
+export function fetchState(name: string): Promise<StateSnapshot> {
+  return getJSON<StateSnapshot>(`/view/api/state/${encodeURIComponent(name)}`);
+}
+
+export function fetchVersion(): Promise<VersionInfo> {
+  return getJSON<VersionInfo>("/view/api/version");
+}

cmd/frontend/web/src/api/sse.ts

@@ -0,0 +1,92 @@
+import type {
+  BackendEventPayload,
+  BrowserEvent,
+  FrontendEventPayload,
+  LogEventPayload,
+  MaglevdStatusPayload,
+} from "../types";
+import { fetchAllState } from "./rest";
+import {
+  applyBackendEffectiveWeight,
+  applyBackendTransition,
+  applyFrontendTransition,
+  applyMaglevdStatus,
+  replaceAll,
+} from "../stores/state";
+import { pushEvent } from "../stores/events";
+
+// openEventStream wires the SPA to /view/api/events. EventSource auto-
+// reconnects with the Last-Event-ID header set, which the Go broker uses
+// to replay events from its 30s ring buffer. A "resync" event tells us to
+// refetch full state and redraw.
+export function openEventStream(): EventSource {
+  const es = new EventSource("/view/api/events");
+
+  es.onmessage = (msg) => {
+    try {
+      const ev = JSON.parse(msg.data) as BrowserEvent;
+      dispatch(ev);
+    } catch (err) {
+      console.error("sse parse error", err, msg.data);
+    }
+  };
+
+  // "resync" is emitted as a named event so we can listen for it
+  // without it going through the default onmessage dispatch.
+  es.addEventListener("resync", async () => {
+    try {
+      const snaps = await fetchAllState();
+      replaceAll(snaps);
+    } catch (err) {
+      console.error("resync refetch failed", err);
+    }
+  });
+
+  es.onerror = (err) => {
+    // EventSource handles reconnection on its own — just log.
+    console.debug("sse error, browser will reconnect", err);
+  };
+
+  return es;
+}
+
+function dispatch(ev: BrowserEvent) {
+  pushEvent(ev);
+  switch (ev.type) {
+    case "backend":
+      applyBackendTransition(ev.maglevd, ev.payload as BackendEventPayload);
+      break;
+    case "frontend":
+      applyFrontendTransition(ev.maglevd, ev.payload as FrontendEventPayload);
+      break;
+    case "maglevd-status":
+      applyMaglevdStatus(ev.maglevd, ev.payload as MaglevdStatusPayload);
+      break;
+    case "log":
+      applyLogEvent(ev.maglevd, ev.payload as LogEventPayload);
+      break;
+  }
+}
+
+// applyLogEvent surfaces the few log messages that carry data we want to
+// reflect in the store. Probe-start/probe-done drive the heartbeat and are
+// handled by BackendRow watching the events signal directly; here we only
+// react to VPP LB sync mutations so the effective weight column updates
+// live when a backend is disabled, enabled, or reweighted.
+function applyLogEvent(maglevd: string, p: LogEventPayload) {
+  if (!p.msg.startsWith("vpp-lb-sync-as-")) return;
+  const attrs = p.attrs ?? {};
+  const address = attrs.address;
+  if (!address) return;
+  switch (p.msg) {
+    case "vpp-lb-sync-as-added":
+      applyBackendEffectiveWeight(maglevd, address, Number(attrs.weight ?? 0));
+      break;
+    case "vpp-lb-sync-as-removed":
+      applyBackendEffectiveWeight(maglevd, address, 0);
+      break;
+    case "vpp-lb-sync-as-weight-updated":
+      applyBackendEffectiveWeight(maglevd, address, Number(attrs.to ?? 0));
+      break;
+  }
+}