fix(firmware): defensive node_id capture prevents runtime clobber (#390)

Users on multi-node ESP32 deployments have been reporting for months
that their provisioned `node_id` reverts to the Kconfig default of `1`
in UDP frames and the `csi_collector` init log, despite boot showing:

    nvs_config: NVS override: node_id=4
    main: ESP32-S3 CSI Node (ADR-018) - Node ID: 4
    csi_collector: CSI collection initialized (node_id=1, channel=11)

See #232, #375, #385, #386, #390. The root memory-corruption path for
the `g_nvs_config.node_id` byte has not been definitively isolated
(does not reproduce on my attached ESP32-S3 running current source
and the v0.6.0 release binary), but the UDP frame header can be made
tamper-proof regardless:

1. `csi_collector_init()` now captures `g_nvs_config.node_id` into a
   module-local `static uint8_t s_node_id` at init time.
2. `csi_serialize_frame()` reads `buf[4]` from `s_node_id`, not from
   the global - so any later corruption of `g_nvs_config` cannot
   affect outgoing CSI frames.
3. All other consumers (`edge_processing.c` x3, `wasm_runtime.c`,
   `display_ui.c`, `main.c swarm_bridge_init`) now go through a new
   `csi_collector_get_node_id()` accessor instead of reading the
   global directly.
4. A canary at end-of-init logs `WARN` if `g_nvs_config.node_id`
   already diverges from the captured value - this will pinpoint
   the corruption path if it happens on a user's device.

Hardware validation on attached ESP32-S3 (COM8):
  - NVS loads node_id=2
  - Boot log: `main: ... Node ID: 2`
  - NEW log: `csi_collector: Captured node_id=2 at init (defensive
    copy for #232/#375/#385/#390)`
  - Init log: `csi_collector: CSI collection initialized (node_id=2)`
  - UDP frame byte[4] = 2 (verified via socket sniffer, 15/15 packets)

This is defense in depth - it shields the UDP frame from whatever
upstream bug is clobbering the struct. When a user hits the original
bug, the canary WARN will help isolate the root cause.

Refs #232 #375 #385 #386 #390

Co-Authored-By: claude-flow <ruv@ruv.net>

firmware/esp32-csi-node/main/main.c

@@ -267,7 +267,7 @@ void app_main(void)
         strncpy(swarm_cfg.seed_url, g_nvs_config.seed_url, sizeof(swarm_cfg.seed_url) - 1);
         strncpy(swarm_cfg.seed_token, g_nvs_config.seed_token, sizeof(swarm_cfg.seed_token) - 1);
         strncpy(swarm_cfg.zone_name, g_nvs_config.zone_name, sizeof(swarm_cfg.zone_name) - 1);
-        swarm_ret = swarm_bridge_init(&swarm_cfg, g_nvs_config.node_id);
+        swarm_ret = swarm_bridge_init(&swarm_cfg, csi_collector_get_node_id());
         if (swarm_ret != ESP_OK) {
             ESP_LOGW(TAG, "Swarm bridge init failed: %s", esp_err_to_name(swarm_ret));
         }