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, 5 insertions, 52 deletions

diff --git a/main/display.c b/main/display.c
index 13babdb..a704891 100644
--- a/main/display.c
+++ b/main/display.c
@@ -101,13 +101,8 @@ enum {
 #define TC_PPI_STARTPPI           0x0104
 #define TC_DSI_STARTDSI           0x0204
 
-// Shadow of esp_lcd_dsi_bus_t. The IDF private struct is
-//   { int bus_id; mipi_dsi_hal_context_t hal; esp_pm_lock_handle_t pm_lock; }
-// (esp-idf/components/esp_lcd/dsi/mipi_dsi_priv.h). We only need bus_id +
-// hal to reach the LL/HAL APIs, so the trailing pm_lock is fine to ignore.
-// If the IDF layout shifts in a future release this is the place that
-// breaks first — guarded by ESP-IDF version is overkill until it actually
-// breaks.
+// Mirror of esp_lcd_dsi_bus_t (esp-idf/components/esp_lcd/dsi/mipi_dsi_priv.h)
+// — needed to reach the HAL/LL APIs for the TC358762 bridge init.
 typedef struct {
     int                    bus_id;
     mipi_dsi_hal_context_t hal;
@@ -424,12 +419,9 @@ esp_err_t display_wake(void)
     return err;
 }
 
-// RGB565 → RGB888 in B,G,R byte order. The ESP32-P4 DSI engine + TC358762
-// bridge pipeline expects BGR on the wire for the panel's RGB channels —
-// writing R first paints pure blue as solid red on the panel (verified
-// with the M5 self-test: decoded buffer holds 0x001F, panel showed red).
-// Symmetric pixels (white text on black) don't expose this; saturated
-// channel inputs do.
+// RGB565 → 24-bit pixel in B,G,R memory order — what the DSI engine +
+// TC358762 + Pi panel pipeline expects (the "RGB888" label refers to the
+// MIPI wire bit order, not byte position in PSRAM).
 static inline void rgb565_to_rgb888(uint16_t px, uint8_t *out)
 {
     out[0] = (uint8_t)(( px        & 0x1F) * 255 / 31);  // B
@@ -437,22 +429,6 @@ static inline void rgb565_to_rgb888(uint16_t px, uint8_t *out)
     out[2] = (uint8_t)(((px >> 11) & 0x1F) * 255 / 31);  // R
 }
 
-esp_err_t display_fill(uint16_t rgb565)
-{
-    if (!s_up || !s_rot_buf) return ESP_ERR_INVALID_STATE;
-    uint8_t rgb[3];
-    rgb565_to_rgb888(rgb565, rgb);
-    size_t n = PANEL_H_ACTIVE * PANEL_V_ACTIVE;
-    for (size_t i = 0; i < n; ++i) {
-        s_rot_buf[i * 3 + 0] = rgb[0];
-        s_rot_buf[i * 3 + 1] = rgb[1];
-        s_rot_buf[i * 3 + 2] = rgb[2];
-    }
-    return esp_lcd_panel_draw_bitmap(s_panel, 0, 0,
-                                     PANEL_H_ACTIVE, PANEL_V_ACTIVE,
-                                     s_rot_buf);
-}
-
 esp_err_t display_present_rgb565(const uint16_t *src,
                                  uint16_t        src_w,
                                  uint16_t        src_h)
@@ -463,8 +439,6 @@ esp_err_t display_present_rgb565(const uint16_t *src,
     viewport_orientation_t orient = viewport_state_get()->orientation;
     viewport_state_unlock();
 
-    if (!s_rot_buf) return ESP_ERR_INVALID_STATE;
-
     if (orient == VIEWPORT_ORIENTATION_LANDSCAPE) {
         if (src_w != PANEL_H_ACTIVE || src_h != PANEL_V_ACTIVE)
             return ESP_ERR_INVALID_SIZE;
@@ -489,24 +463,3 @@ esp_err_t display_present_rgb565(const uint16_t *src,
                                      PANEL_H_ACTIVE, PANEL_V_ACTIVE,
                                      s_rot_buf);
 }
-
-esp_err_t display_test_pattern(void)
-{
-    if (!s_up || !s_rot_buf) return ESP_ERR_INVALID_STATE;
-    static const uint16_t bars[8] = {
-        0xFFFF, 0xFFE0, 0x07FF, 0x07E0,
-        0xF81F, 0xF800, 0x001F, 0x0000,
-    };
-    const int bar_w = PANEL_H_ACTIVE / 8;
-    for (int y = 0; y < PANEL_V_ACTIVE; ++y) {
-        for (int x = 0; x < PANEL_H_ACTIVE; ++x) {
-            int b = x / bar_w;
-            if (b > 7) b = 7;
-            size_t idx = ((size_t)y * PANEL_H_ACTIVE + x) * 3;
-            rgb565_to_rgb888(bars[b], &s_rot_buf[idx]);
-        }
-    }
-    return esp_lcd_panel_draw_bitmap(s_panel, 0, 0,
-                                     PANEL_H_ACTIVE, PANEL_V_ACTIVE,
-                                     s_rot_buf);
-}