Rebuild the framework with smaller TLS buffers (ADR 0006)

custom_sdkconfig makes pioarduino regenerate the ESP-IDF libraries:
asymmetric TLS buffers (16 KB in, 4 KB out) and dynamic buffers that
free handshake-only data once connected. The rebuild also follows the
board: no PSRAM, 8 MB flash. The project now carries the partition
table the hybrid build needs (identical to the framework's: the app
slots must not move under updates over the air). Generated files are
ignored.

Debug Build, GNSS on, IRC on TLS: 78 KB free and a 59 KB low (M2 began
at 31 KB and 12.6 KB; the floor is 40 KB). The full stress set passes
on it: refused installs over Wi-Fi and from SD, put/get, screenshot,
core dump decode, Probation; under all of it at once, the low is 46 KB.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

docs/adr/0006-framework-rebuilt-for-smaller-tls-buffers.md

@@ -0,0 +1,19 @@
+# The framework is rebuilt with our own SDK settings, for smaller TLS buffers
+
+Arduino-ESP32 ships its ESP-IDF libraries prebuilt, with one `sdkconfig` for every ESP32-S3 board. Its TLS settings give every connection a 16 KB receive buffer and a 16 KB send buffer for its whole life. On a device with no PSRAM and about 340 KB of RAM, an IRC connection over TLS left a 12.6 KB low in M2, against a 40 KB floor.
+
+Those settings are compiled into the libraries, so changing them means rebuilding them. pioarduino supports this as a "hybrid compile": `custom_sdkconfig` in `platformio.ini` lists the settings, and the build regenerates the framework's libraries from ESP-IDF (the same 5.5.5 the prebuilt ones come from) before building the app. We set:
+
+- `MBEDTLS_ASYMMETRIC_CONTENT_LEN`, with 16 KB to receive (servers send full TLS records) and **4 KB to send** (IRC lines are short): 12 KB less per connection.
+- `MBEDTLS_DYNAMIC_BUFFER`, `DYNAMIC_FREE_CONFIG_DATA`, `DYNAMIC_FREE_CA_CERT`: buffers allocated when needed, and handshake-only data (the CA chain) freed once connected.
+
+The rebuild also follows the board definition instead of the generic one: PSRAM support is off (the Cardputer ADV has none) and the flash size is 8 MB.
+
+## Consequences
+
+- With IRC connected over TLS, a Debug Build has 78 KB free and a 59 KB low (it was 31 KB and 12.6 KB), and 46 KB at the lowest under the heaviest combined load measured (IRC, two refused installs, a 1.6 MB put and get).
+- The first build after a fresh checkout, or after changing `custom_sdkconfig`, takes about 4 minutes instead of 45 s: it downloads ESP-IDF into the PlatformIO volume and compiles it. Later builds reuse it.
+- Everything the firmware depends on was checked in the regenerated `sdkconfig`: app rollback, core dumps to flash (ELF), the 5 s task watchdog, FreeRTOS run-time stats, the certificate bundle.
+- The project now owns its partition table (`default_8MB.csv`, identical to the framework's), which the hybrid build requires. Changing it would break updates over the air: the app slots must stay where they are.
+- Generated files (`sdkconfig.*`, `managed_components/`, `.dummy/`) are ignored by git.
+- A TLS server that sends records over 16 KB would still fail, as before; one that needs us to send records over 4 KB would now fail. Neither happens with IRC.

docs/milestones/M2.md

@@ -1,12 +1,21 @@
 # M2 — GNSS
 
-**Status:** steps 1–6 done on the device (branch `m2`). Open: the heap floor during a TLS handshake (below).
+**Status:** done on the device (branch `m2`): every "Done when" item below is met.
 
 ## Measured
 
 - **Cold start** (`$PCAS10,2`) to a 3D Fix, by a window: **73 s**, 5 satellites used of 8 in view. A restart of the ESP32 alone keeps the receiver's Fix (the Cap stays powered).
 - By a window: 3D Fix from GPS, GLONASS, Galileo and BeiDou, up to 14 of 17 satellites used, HDOP 1.0–1.3.
-- **Heap, Debug Build, GNSS on:** with IRC on TLS, first 33 KB free and a 12.6 KB low (floor 40 KB; v0.2.1 had a 79 KB low). Not a GNSS cost: the OTA and Debug Build work added task stacks, a console ring, serial buffers, mDNS and two TCP servers. **Trimmed by measurement:** each task's peak stack was measured through its worst case (an ECDSA-checked install over Wi-Fi and from SD, get/put, a core dump fetch, an IRC TLS handshake), then stacks were set to peak plus about 2 KB: loop 8→6 KB, update 8→5, storage 10→6, irc 8→6; the console ring 6→4 KB, serial TX 2→1 KB, GNSS UART 1 KB→512 B. **After:** 46 KB free with IRC connected, an 18 KB low. The steady state clears the floor; the TLS handshake peak doesn't yet. Next candidates: mbedTLS dynamic buffers (custom sdkconfig), one network task for the update and debug listeners, mDNS on demand.
+- **Heap, Debug Build, GNSS on, IRC on TLS** (floor 40 KB; v0.2.1 had a 79 KB low):
+
+  | | Free | Lowest |
+  |---|---|---|
+  | Start of M2 | 31 KB | 12.6 KB |
+  | Stacks and buffers trimmed by measurement | 46 KB | 18 KB |
+  | mDNS removed | 54 KB | 34 KB |
+  | Framework rebuilt with smaller TLS buffers (ADR 0006) | **78 KB** | **59 KB** |
+
+  Under the heaviest combined load measured (IRC, two refused installs, a 1.6 MB put and get), the low is 46 KB. The cost had come mostly from the OTA and Debug Build work, not GNSS. Stacks were set to measured peak plus about 2 KB (loop 6 KB, update 5, storage 6, irc 6); after a TLS handshake the irc task has 1.7 KB left. IRC can now be stopped by hand (`/quit` in any state, `irc stop`), which frees its TLS memory.
 
 **Goal:** the device knows where it is and what time it is without a network: a GNSS Service in the background, a GNSS App with the position and a sky view of the satellites, the clock set from satellites when there's no NTP, and Tracks recorded to the SD card.