docs(build): add BUILDING.md, pyproject.toml, and radio presets

BUILDING.md

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+# Building EdgeTX
+
+This guide covers how to build EdgeTX firmware, the SDL simulator, and the
+Companion application from source. For platform-specific environment setup
+(installing toolchains, Qt, etc.), see the guides in [`docs/building/`](docs/building/).
+
+## Prerequisites
+
+| Component | Required for | Notes |
+|-----------|-------------|-------|
+| **CMake >= 3.21** | Everything | |
+| **arm-none-eabi-gcc** | Firmware | 14.2.rel1 recommended |
+| **Python >= 3.10** | Firmware, Companion | Code generation at build time |
+| **Qt 6** | Companion | 6.9+ recommended |
+| **SDL2** | Simulator | |
+| **Ninja** (optional) | Everything | Faster than Make |
+
+### Python dependencies
+
+The build invokes Python scripts for code generation (YAML parsers, hardware
+definitions, bitmap encoding, etc.). Install the required packages with
+[uv](https://docs.astral.sh/uv/):
+
+```bash
+uv sync
+source .venv/bin/activate   # CMake needs the venv activated to find Python
+```
+
+The virtual environment must stay activated when running CMake, so that the
+build can find the installed packages. On Windows, use `.venv\Scripts\activate`
+instead.
+
+## Quick start with CMake Presets
+
+The repository ships CMake presets for all supported radios and build types.
+These work in VSCode (CMake Tools), CLion, and the command line.
+
+### Firmware
+
+```bash
+# Configure for a specific radio
+cmake --preset tx16s
+
+# Build
+cmake --build build/tx16s --target firmware
+```
+
+Replace `tx16s` with any supported radio preset. Run `cmake --list-presets` to
+see all available presets. A few examples:
+
+| Preset | Radio |
+|--------|-------|
+| `tx16s` | RadioMaster TX16S |
+| `tx16smk3` | RadioMaster TX16S MK3 |
+| `boxer` | RadioMaster Boxer |
+| `pocket` | RadioMaster Pocket |
+| `zorro` | RadioMaster Zorro |
+| `t20` | Jumper T20 |
+| `t14` | Jumper T14 |
+| `tpro` | Jumper T-Pro |
+| `x9dp2019` | FrSky X9D+ 2019 |
+| `x10` | FrSky X10 |
+| `nv14` | FlySky NV14 |
+| `pl18` | FlySky PL18 |
+
+
+### Simulator
+
+```bash
+cmake --preset simu -DPCB=X10 -DPCBREV=TX16S
+cmake --build build/simu --target simu
+```
+
+### Companion
+
+Companion embeds a radio simulator that uses WASM plugins — one per radio
+target. With presets, this requires two build directories: one for the WASM
+plugins and one for Companion itself.
+
+Build the WASM plugins for all radios:
+
+```bash
+tools/build-wasm-modules.sh
+
+# or limit to specific radios
+FLAVOR="tx16s;boxer" tools/build-wasm-modules.sh
+```
+
+Then build Companion:
+
+```bash
+cmake --preset companion
+cmake --build build/companion --target companion --parallel
+```
+
+When developing for a specific radio, you can build just that one WASM plugin
+using the superbuild instead (see [Manual CMake](#manual-cmake-without-presets)
+below):
+
+```bash
+cmake -S . -B build-tx16s -DPCB=X10 -DPCBREV=TX16S
+cmake --build build-tx16s --target wasi-module --parallel
+cmake --build build-tx16s --target companion --parallel
+```
+
+### Tests
+
+```bash
+cmake --preset simu -DPCB=X10 -DPCBREV=TX16S
+cmake --build build/simu --target tests-radio
+```
+
+## Manual CMake (without presets)
+
+If you prefer to invoke CMake directly, the superbuild takes care of toolchain
+selection and sub-build configuration automatically:
+
+```bash
+# configure
+cmake -S . -B build-tx16s -DPCB=X10 -DPCBREV=TX16S
+
+# build firmware
+cmake --build build-tx16s --target firmware --parallel
+
+# build simulator (same build directory)
+cmake --build build-tx16s --target simu --parallel
+
+# run tests
+cmake --build build-tx16s --target tests-radio --parallel
+```
+
+## Key CMake variables
+
+| Variable | Default | Description |
+|----------|---------|-------------|
+| `PCB` | (required) | Target radio family (X7, X10, PL18, ...) |
+| `PCBREV` | (varies) | Radio variant within a PCB family (TX16S, BOXER, ...) |
+| `CMAKE_BUILD_TYPE` | | `Release` or `Debug` (native builds only; firmware ELF always includes debug symbols, stripped for .bin/.uf2) |
+| `TRANSLATIONS` | EN | Firmware language (EN, FR, DE, ES, ...) |
+| `DEFAULT_MODE` | | Stick mode 1-4 |
+| `LUA` | ON | Lua scripting support |
+
+For the full list, see [`docs/building/compilation-options.md`](docs/building/compilation-options.md).
+
+## FetchContent dependency cache
+
+Downloaded third-party dependencies (googletest, yaml-cpp, imgui, etc.) are
+cached in `.cache/fetchcontent/` at the repository root. This cache is shared
+across all build directories so that switching targets or doing clean rebuilds
+does not re-download dependencies.
+
+To override the cache location, set the `FETCHCONTENT_BASE_DIR` environment
+variable or pass it as a CMake variable:
+
+```bash
+export FETCHCONTENT_BASE_DIR=~/edgetx-deps
+cmake --preset tx16s
+```
+
+## CI builds
+
+CI uses the scripts in `tools/`:
+
+| Script | Purpose |
+|--------|---------|
+| `build-gh.sh` | Firmware release builds (all targets) |
+| `commit-tests.sh` | Debug builds with warnings-as-errors |
+| `build-companion.sh` | Companion app (Linux, macOS, Windows) |
+| `build-wasm-modules.sh` | WebAssembly simulator modules |
+| `build-common.sh` | Shared helpers and target definitions |
+
+These scripts use `FLAVOR` environment variables and are tailored for GitHub
+Actions. For local development, prefer the CMake presets above.