Tidy: delete dead code, inline single-use helpers, fix double-lock

Code-review pass after the M5 colour fix. -126 net lines (-215 / +89).

Dead code removed:
- display_fill, display_test_pattern — M3 bring-up self-tests, no callers
- net_eth_is_up — no callers
- nvs_config_reset — no callers (factory-reset gesture was removed earlier;
  NVS wipe now goes through `idf.py erase-flash`)

Helpers inlined or collapsed (each had one call site):
- state_name / orientation_name — ternaries at the cJSON site
- fmt_bytes / fmt_uptime — inline scope in local_screens_show_info
- expected_dims (http_api) + effective_dims (local_screens) — merged into
  viewport_state_effective_dims() in viewport_state, one canonical source

state_machine cleanup:
- arm_idle_timer_unlocked / disarm_idle_timer collapsed to one arm_idle_timer(ms)
  that takes the snapshotted timeout (ms==0 disables). Removes the misleading
  name and the second viewport_state lock acquisition per painted frame.
- state_machine_set + state_machine_frame_painted now snapshot state + idle_ms
  under one lock.

mdns_service cleanup:
- snapshot_state / apply_hostname / apply_txt collapsed into a single
  apply_state(include_hostname) helper; mdns_service_start grabs hostname
  once for hostname_set + log instead of going under the lock three times.

http_api cleanup:
- POST /config brightness-changed path uses the local `bright` it already
  validated instead of re-locking + re-reading st->brightness.
- Trimmed verbose bring-up comments (stack-size justification, M5 saga,
  DSI shadow struct) to one line each — kept the load-bearing facts.

Smoke-tested on hardware: boot clean, display + JPEG + touch all up
([DJT]), info screen renders, no crashes.

1 files changed, 31 insertions, 50 deletions

diff --git a/main/mdns_service.c b/main/mdns_service.c
index d389161..21cd464 100644
--- a/main/mdns_service.c
+++ b/main/mdns_service.c
@@ -14,48 +14,30 @@ static const char *SERVICE   = "_scrypted-viewport";
 static const char *PROTO     = "_tcp";
 static const uint16_t PORT   = 80;
 
-// Pull the current hostname + TXT values from viewport_state under the lock.
-static void snapshot_state(char *hostname, size_t host_cap,
-                           const char **resolution,
-                           const char **orientation,
-                           char *name, size_t name_cap)
-{
-    viewport_state_lock();
-    viewport_state_t *st = viewport_state_get();
-
-    if (st->viewport_name[0]) {
-        snprintf(hostname, host_cap, "viewport-%s", st->viewport_name);
-    } else {
-        snprintf(hostname, host_cap, "viewport");
-    }
-    *resolution  = viewport_state_resolution_str();
-    *orientation = (st->orientation == VIEWPORT_ORIENTATION_PORTRAIT)
-                       ? "portrait" : "landscape";
-    snprintf(name, name_cap, "%s", st->viewport_name[0] ? st->viewport_name : "");
-
-    viewport_state_unlock();
-}
-
-static esp_err_t apply_hostname(void)
+// Pull hostname + TXT values from viewport_state under one lock acquisition,
+// then push them at the mDNS service. mdns_service_add() rejects with
+// INVALID_ARG if the hostname isn't set, so order matters on first apply.
+static esp_err_t apply_state(bool include_hostname)
 {
     char hostname[80];
-    const char *resolution, *orientation;
     char name[64];
-    snapshot_state(hostname, sizeof(hostname),
-                   &resolution, &orientation,
-                   name, sizeof(name));
-    return mdns_hostname_set(hostname);
-}
-
-static esp_err_t apply_txt(void)
-{
-    char hostname[80];
     const char *resolution, *orientation;
-    char name[64];
-    snapshot_state(hostname, sizeof(hostname),
-                   &resolution, &orientation,
-                   name, sizeof(name));
 
+    viewport_state_lock();
+    viewport_state_t *st = viewport_state_get();
+    snprintf(hostname, sizeof(hostname), "viewport%s%s",
+             st->viewport_name[0] ? "-" : "",
+             st->viewport_name[0] ? st->viewport_name : "");
+    snprintf(name, sizeof(name), "%s",
+             st->viewport_name[0] ? st->viewport_name : "");
+    resolution  = viewport_state_resolution_str();
+    orientation = (st->orientation == VIEWPORT_ORIENTATION_PORTRAIT)
+                      ? "portrait" : "landscape";
+    viewport_state_unlock();
+
+    if (include_hostname) {
+        ESP_RETURN_ON_ERROR(mdns_hostname_set(hostname), TAG, "hostname_set");
+    }
     mdns_txt_item_t txt[] = {
         { "version",     VIEWPORT_VERSION },
         { "resolution",  resolution },
@@ -68,21 +50,21 @@ static esp_err_t apply_txt(void)
 
 esp_err_t mdns_service_start(void)
 {
-    // Order matters: mdns_service_add() rejects with INVALID_ARG if the
-    // hostname isn't set yet. So: init -> hostname -> service_add -> TXT.
-    ESP_RETURN_ON_ERROR(mdns_init(),           TAG, "mdns_init");
-    ESP_RETURN_ON_ERROR(apply_hostname(),       TAG, "hostname_set");
+    char hostname[80];
+    viewport_state_lock();
+    viewport_state_t *st = viewport_state_get();
+    snprintf(hostname, sizeof(hostname), "viewport%s%s",
+             st->viewport_name[0] ? "-" : "",
+             st->viewport_name[0] ? st->viewport_name : "");
+    viewport_state_unlock();
+
+    ESP_RETURN_ON_ERROR(mdns_init(),                       TAG, "mdns_init");
+    ESP_RETURN_ON_ERROR(mdns_hostname_set(hostname),        TAG, "hostname_set");
     ESP_RETURN_ON_ERROR(
         mdns_service_add(NULL, SERVICE, PROTO, PORT, NULL, 0),
         TAG, "service_add");
-    ESP_RETURN_ON_ERROR(apply_txt(),            TAG, "txt_set");
+    ESP_RETURN_ON_ERROR(apply_state(false),                 TAG, "txt_set");
 
-    char hostname[80];
-    const char *resolution, *orientation;
-    char name[64];
-    snapshot_state(hostname, sizeof(hostname),
-                   &resolution, &orientation,
-                   name, sizeof(name));
     ESP_LOGI(TAG, "mDNS up — %s.local advertising %s.%s.local on :%u",
              hostname, SERVICE, PROTO, PORT);
     return ESP_OK;
@@ -90,6 +72,5 @@ esp_err_t mdns_service_start(void)
 
 esp_err_t mdns_service_refresh(void)
 {
-    ESP_RETURN_ON_ERROR(apply_hostname(), TAG, "hostname_set");
-    return apply_txt();
+    return apply_state(true);
 }