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, 0 insertions, 8 deletions

diff --git a/main/display.h b/main/display.h
index 3119a60..9092efb 100644
--- a/main/display.h
+++ b/main/display.h
@@ -27,14 +27,6 @@ esp_err_t display_set_brightness(uint8_t brightness_0_100);
 esp_err_t display_sleep(void);
 esp_err_t display_wake(void);
 
-// Paint a solid RGB565 color to the entire framebuffer + flush.
-// Used by M3 for the test pattern and by local_screens for the IP and
-// loading screens (M8).
-esp_err_t display_fill(uint16_t rgb565);
-
-// Show a deterministic test pattern (vertical color bars). M3 acceptance.
-esp_err_t display_test_pattern(void);
-
 // Blit an RGB565 source image to the panel, applying the current
 // orientation. Source dimensions must match the effective resolution:
 //   portrait  -> src is 480x800 (rotated 90° CW into the 800x480 panel)