XeWeCore 3.1.2

XeWeCore is the modular firmware core for ESP32, as one Arduino library: a serial console, a
"$group command" command line, typed NVS storage, small helpers and a module framework. Use it on
its own from the Arduino IDE, or through an xewe-os project (github.com/xewe-labs/xewe-os), whose
tools pull it in together with the ready-made modules.

3.1.2 follows 3.1.1 and settles the form of the configuration file. No behaviour changes.

CHANGED SINCE 3.1.1

- src/XeWeCore/Config.h has two sections, each between a start and an end marker line:
  `/* ---- start default runtime values ---- */` ... `/* ---- end default runtime values ---- */`
  and `/* ---- start compile time defined constants ---- */` ...
  `/* ---- end compile time defined constants ---- */`. The first-boot section is gone: a value
  the first boot asks for is a default runtime value whose group defines <NAME>_PROMPT 1 and whose
  comment says "asked at first boot". The device name is the core's only such value.
- Every text line of Config.h is a /* */ block comment; // remains only on the SPDX header lines.
  A project's Config.h, generated by xewe-os-tools 3.1.5, keeps that form, frames each block with
  /****/ bars and uses // only in front of commented-out definitions.
- doc/config.md describes the two sections, the block-comment form and the project-side catalogue.

REMOVED

- XEWE_CORE_PRINT_BANNER: the banner always prints. OsConfig::print_banner keeps a literal default
  of true. A build that still defines the old name gets an unused define and no error.

INSTALL

- Arduino IDE: Library Manager, XeWeCore 3.1.2. Depends on ArduinoJson >=7.0.0 (tested 7.4.3).
- xewe-os projects: [core] ref = "3.1.2" in xewe.toml, then ./setup.sh. Built and tested with
  xewe-os-tools 3.1.3, arduino-cli 1.5.1 and esp32 core 3.3.12.

TESTING

What was tested, on which tree and with which result: release_card.md, generated by
scripts/release.sh --prepare 3.1.2 and attached to this release. Board tests ran on an ESP32-C3;
the C6 and S3 images were compiled, not run.
