Fix WiFi reconnect bugs, negative-temp rounding, and stale docs

- Clear WIFI_CONNECTED_BIT on disconnect so wifi_is_connected() stops
  reporting a link that is gone; the weather task's reconnect path was
  dead code without it.
- After the 8 fast retries are spent, retry every 30s via an esp_timer
  forever instead of giving up until power cycle. FAIL_BIT now only
  unblocks the boot-time wait and is cleared on reconnect.
- Start the weather task even when the boot-time connect times out --
  the background retries may still bring WiFi up later. Only an empty
  SSID skips it.
- Round temperatures with lroundf(): the old +0.5f-and-truncate rounded
  toward zero below 0degF (-10.6 displayed as -10).
- Drop the stale single-framebuffer "Partial-mode flush" comment in
  display_rgb.c, fix the README paragraph claiming the bounce buffer is
  disabled (it is enabled and load-bearing), and correct the
  sdkconfig.defaults header to N16R8/16MB.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -130,7 +130,10 @@ Two 800×480×2 framebuffers (750 KB each) live in PSRAM. The 8 MB **octal**
 PSRAM at 80 MHz is required — QSPI cannot sustain the RGB refresh.
 
 Do **not** enable `CONFIG_SPIRAM_FETCH_INSTRUCTIONS` / `CONFIG_SPIRAM_RODATA`:
-they contend with the panel for PSRAM bandwidth. Do **not** enable the RGB
-bounce buffer either — its refill ISR reads PSRAM, which shares the cache with
-flash on the S3, so any NVS write faults it with *"Cache disabled but cached
-memory region accessed"*.
+they contend with the panel for PSRAM bandwidth. The bounce buffer **is**
+enabled (the panel cannot stream straight from PSRAM), which is exactly why
+flash writes are dangerous while the panel runs: the refill ISR reads PSRAM,
+which shares the cache with flash on the S3, so an NVS write faults it with
+*"Cache disabled but cached memory region accessed"*. All flash writes happen
+before `display_init()` and `CONFIG_ESP_WIFI_NVS_ENABLED` is off to keep the
+WiFi stack from writing later.