Work through the remaining review items in TODO.md

Keep the HTTP session alive between weather polls (TODO #1). Re-creating the
client each time forced a fresh TLS handshake inside the window where the UI
is frozen. Measured on hardware: the fetch went from ~1.3s to ~80ms, so the
pause is no longer long enough to matter once thermostat controls exist. The
timeout drops from 30s to 10s once a session is warm, since only a cold
handshake needs the longer budget. Any transport error or non-200 tears the
session down so a half-open connection cannot poison every later poll.

The response buffer is now reset per request rather than on
HTTP_EVENT_ON_CONNECTED -- a kept-alive connection does not re-fire that
event, so the buffer would have accumulated across polls.

Indicate stale readings (TODO #2). After two consecutive failures the
temperature dims and the status line shows the age. Age comes from tick
counts, not weather_data_t.last_update: there is no SNTP client here, so
time(NULL) starts at the epoch and any age derived from it is meaningless.

Gate the touch readout behind UI_DEBUG_TOUCH, default off (TODO #3). Kept the
note about LVGL not bubbling events to parents -- it documents a real trap.

Delete main/utils (TODO #4); it had no callers outside itself. Also refreshed
project_structure.txt, which was additionally missing i2c_bus and backlight.

Lay out two 4MB OTA slots (TODO #5) while no devices are fielded, and document
in the README that an OTA download cannot run while the panel is scanning --
the flash write faults the bounce-buffer ISR.

Log the truncation warning once per response instead of once per chunk, and
record the weather task's stack high-water mark (TODO #6): ~2400 bytes spare
of 6144.

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

README.md

@@ -25,7 +25,6 @@ implemented yet.
 | `main/ui/ui_manager.c` | Screen layout and update entry points |
 | `main/wifi`            | WiFi station bring-up |
 | `main/api`             | Ambient Weather REST client |
-| `main/utils`           | Small helpers |
 
 ## Setup
 
@@ -124,6 +123,21 @@ High Performance Mode and a DC-aware bootloader.
   is running faults it with *"Cache disabled but cached memory region
   accessed"*.
 
+## Partitions
+
+Two 4 MB OTA slots (`ota_0` / `ota_1`) with `otadata`, laid out in
+`partitions.csv`. The app is ~1.4 MB, so the slots are generous — but the
+layout is fixed once devices are fielded, and re-partitioning later means
+reflashing each one over USB. ~7.9 MB of the 16 MB is left unallocated;
+adding a partition to free space is safe, moving an existing one is not.
+
+> **OTA is not yet implemented, and it is not a drop-in.** Writing an OTA image
+> is a flash write, and the bounce-buffer refill ISR reads PSRAM, which shares
+> the cache with flash on the S3 (see Memory notes). Downloading an update
+> while the panel is scanning **will** fault that ISR. Any implementation must
+> stop the panel for the download and write, or reboot into a dedicated
+> updater mode first.
+
 ## Memory notes
 
 Two 800×480×2 framebuffers (750 KB each) live in PSRAM. The 8 MB **octal**