Add Heltec CubeCell AB02S (ASR6502) companion: USB/UART only, legacy board 🤖🤖

[kim7tin]

Refs #2017. Related: #1632 (crypto de-duplication would free more flash on this part).

Most of the implementation was done by an AI coding agent (Claude) working from my ideas and direction; it was
tested on real hardware as described below.

What this is

A companion firmware for the Heltec CubeCell GPS-6502 (HTCC-AB02S). This is a legacy board with very small
resources: ASR6502 (Cortex-M0+), 128 KB flash, 16 KB RAM, a fixed 2 KB main stack, SX1262, Air530Z GPS and a
0.96" SSD1306 OLED.

• USB/UART only. The board has no BLE and no WiFi; the app connects over the USB serial port.
• The node does not generate its own identity. Import a private key from the app first. Until then it answers the
  app with a placeholder identity derived from the chip ID and never transmits.
• Limits: 12 contacts in RAM; up to 3 of them are kept across reboots (the ones the app adds or edits;
  removing a contact frees its place), stored direct routes up to 16 bytes (longer routes are
  not stored and that contact is reached by flood), offline queue of 3 messages, 1 group channel, no text CLI.
  Settings kept across reboots: identity, name, radio parameters and TX power, GPS on/off and interval, and the
  companion settings (multi-acks, manual add, telemetry modes, advert location policy, auto-add, path hash mode,
  time zone, airtime and delay factors, CAD/AGC/interference, RX gain) and the 3 pinned contacts. Not kept: the
  manually set location, the default scope, channels, the other contacts.
• OLED: an 8-line status screen (name, key prefix, UTC time, GPS state, lat/lon, battery and contact count,
  frequency/SF). It turns off after 30 s; the USER button wakes it.
• GPS: powered when a position is wanted and switched off after the fix; it sets the clock from RMC. gps and
  gps_interval are exposed through the custom-variable commands.

New env: Heltec_CubeCell_AB02S_companion_radio_usb (platform heltecautomation/heltec-cubecell@1.1.0).

Changes to shared code: all opt-in, other boards build unchanged

Flag	What it does	Default
MAX_ANON_CONTACTS, MAX_PACKET_HASHES, ADVERT_PATH_TABLE_SIZE, COMPANION_PACKET_POOL_SIZE	table sizes become overridable	unchanged
CONTACT_OUT_PATH_SIZE	bytes kept for a contact's direct route; a route that does not fit is not stored	MAX_PATH_SIZE
RX_DELAY_FAST_POW	rx delay without libm pow() (< 1 % error)	off
DISABLE_CLI_RESCUE	no rescue CLI (it prints text on the only serial port and needs a filesystem)	off
DISABLE_COMPANION_CLI	no text CLI; radio settings still use the binary commands	off
CONFIG_SERIALIZER_NOOP	prefs serializer without the text reader/writer (prefs kept as a binary record)	off
ED25519_SMALL_BASE_TABLE	first row of the fixed-base table only, Horner evaluation (~30 KB less flash)	off
IDENTITY_CRYPTO_ON_BIG_STACK	Ed25519 runs on a heap-allocated stack	off
CONTACT_PIN_STORE	companion hooks so a store can keep contacts the app adds/edits (here: 3, in a flash row)	off
SERIAL_FRAME_STRICT	drop frames longer than MAX_FRAME_SIZE instead of executing them truncated; abandon a partial frame after 100 ms with nothing buffered	off

SERIAL_FRAME_STRICT fixes two problems that every USB companion has today: an oversize frame is executed after
being cut short, and a frame cut short by the sender swallows the start of the next one. It might be worth making it
the default; I kept it behind a flag so this PR does not change other boards.

New code lives in src/helpers/asr650x/ (compiled only by this env) and variants/heltec_cubecell_ab02s/.
examples/companion_radio/ gets a flash-row DataStore for boards without a filesystem (FlashDataStore.*, selected
in DataStore.h) and a main.cpp branch for ASR650X_PLATFORM.

Evidence

• Other boards unchanged: RAK_4631_companion_radio_usb, Heltec_v3_companion_radio_usb and
  PicoW_companion_radio_usb report identical RAM and flash sizes before and after this branch (every change to
  shared code is behind a flag, and new files compile only for this env).
• Host tests: pio test -e native 88/88, plus native_noop_config 3/3, native_ed25519_small 3/3 (RFC 8032
  vectors with both tables), native_kiss_modem 8/8.
• AB02S size: the image ends at 129 900 B, about 1 KB below the loader metadata at the top of the 128 KB part.
  Static RAM is 9 408 B (.data + .bss + .ramvectors). Measured on the board: main stack peak 1 360 B (of
  2 048), crypto stack peak 1 760 B, heap peak 3 992 B, so about 870 B of RAM stays free.
• On hardware (companion protocol over USB): 29/29 checks covering identity import and persistence, refusing to
  transmit before a key is imported, framing resync, signatures verified on a PC, adverts imported and rejected;
  15/15 GPS/OLED checks (power control, NMEA filtered to GGA+RMC, settings persist, OLED acknowledges on I2C); route
  storage: a 20-byte route is not stored and a 4-byte route is kept; companion settings set over the protocol survive
  a reboot.
• Over the air with a stock MeshCore companion (nRF52840 + SX1262) at 920.25 MHz / BW 62.5 / SF8 / CR5:
  adverts both ways; a direct message AB02S -> nRF acknowledged over the air (round trip 912 ms); a message
  nRF -> AB02S received; a contact added in the app was still listed after an AB02S reboot.

Build and flashing

• build.sh copies the .cyacd and .hex of ASR650X_PLATFORM envs to out/ (the release workflows build
  every *_companion_radio_usb env). Flash with Heltec's CubeCellFlash (.cyacd) or
  pio run -e Heltec_CubeCell_AB02S_companion_radio_usb -t upload --upload-port <port>.
• run-unit-tests also runs the two new native envs; pr-build-check builds the AB02S env, so a change that
  makes it outgrow 128 KB shows up in CI.

Notes for review

• The SX1262 driver comes from the CubeCell framework (Radio.*), not RadioLib: RadioLib does not fit next to the
  mesh code in 128 KB. Its callbacks run in interrupt context; main-context radio calls detach the DIO1 interrupt
  while they talk to the chip.
• variants/heltec_cubecell_ab02s/c_include_order.py works around a header-name clash on case-insensitive file
  systems (the framework's C aes.h vs rweather/Crypto's AES.h); it only affects C sources of this env.
• src/helpers/asr650x/SmallStrtod.cpp replaces newlib's strtod/strtof/atof on this env only (decimal numbers
  only; newlib's version pulls in ~7 KB).
• The OLED uses the PSoC I2C block directly: Arduino Wire keeps two 128-byte buffers per instance, which this RAM
  budget cannot spare.