docs: rewrite documentation.

add: Nix and Snapcraft support.

docs/reference/build-system.mdx

@@ -0,0 +1,101 @@
+---
+sidebar_label: Build system
+---
+
+# Build System
+
+## Compiler Backends
+
+DCR supports 4 compilation backends. Selection is automatic based on file extension.
+
+| Backend | Files | When used |
+|---------|-------|-----------|
+| `unix_cc` | `.c`, `.cpp`, `.cxx`, `.cc`, `.S` | gcc/clang on Linux/macOS/BSD |
+| `msvc` | `.c`, `.cpp`, `.cxx`, `.cc` | Windows (cl, clang-cl) |
+| `gas` | `.s` | ARM/ARM64 assembler (no preprocessor) |
+| `nasm` | `.asm` | x86/x86_64 assembler |
+
+### Unix CC
+
+- Compiler resolved via `resolve_compiler()`: `DCR_COMPILER` > `DCR_CC` > `[toolchain]` > `build.compiler` > `PATH`
+- Supports `.d` files for header dependency tracking
+- Flags: `-std=`, `-MMD -MF`, `-c -o`, `-I`, `-L`, `-l`
+
+### MSVC
+
+- Supports cl.exe and clang-cl.exe
+- Flags: `/std:`, `/Fo:`, `/Fe:`, `/I`, `/link`
+
+### GAS / NASM
+
+- GAS: `-I`, `-c -o`, `--defsym`
+- NASM: `-I`, `-o`, `-D`, `-f` (format: win64/elf64/macho64/macho32/elf32)
+
+## Incremental Builds
+
+Three levels of incrementality:
+
+1. **mtime** — if output is newer than all inputs, skip
+2. **`.d` files** — header change tracking (including transitive)
+3. **SHA256 fingerprint** — recompile if compiler flags changed (stored in `.dcr_fingerprint`)
+
+## Parallel Compilation
+
+- `thread::scope` for thread pool
+- Atomic task queue (`AtomicU64`)
+- Mutex on stdout (`OUTPUT_MUTEX`)
+- Thread count = `available_parallelism()`
+
+## Build Kinds
+
+| Kind | Type | Path (Linux example) |
+|------|------|---------------------|
+| `bin` | Executable | `target/<triple>/<profile>/<name>` (.exe on Windows) |
+| `staticlib` | Static library | `target/<triple>/<profile>/lib<name>.a` (.lib) |
+| `sharedlib` | Dynamic library | `target/<triple>/<profile>/lib<name>.so` (.dll/.dylib) |
+| `efi` | UEFI application | `target/<triple>/<profile>/<name>.efi` |
+| `elf` | ELF without stdlib | `target/<triple>/<profile>/<name>` |
+| `none` | Compile only, no link | — |
+| `custom` | Full filename+extension control | `target/<triple>/<profile>/<filename>.<extension>` |
+
+## Build Steps
+
+DCR supports pre-build and post-build steps:
+
+- `build.steps` — commands before compilation
+- `build.post_steps` — commands after compilation
+
+Substitutions: `{stem}`, `{in}`, `{out}`, `{profile}`, `{version}`, `{name}`.
+
+Example Qt codegen via build steps:
+
+```toml
+[build.steps]
+moc = "moc {in} -o {out}"
+```
+
+## pkg-config
+
+Automatic lookup (read from raw config):
+
+```toml
+[build]
+pkg_config = ["sdl2", "gl"]
+```
+
+DCR runs `pkg-config --cflags sdl2 gl` and `pkg-config --libs sdl2 gl` and adds results to compiler/linker flags.
+
+## Variable Substitution
+
+Supported variables:
+
+| Variable | Description |
+|----------|-------------|
+| `{version}` | Package version |
+| `{version_major}` | Major version part |
+| `{version_minor}` | Minor version part |
+| `{version_patch}` | Patch version |
+| `{version_suffix}` | Suffix (e.g., `-rc1`) |
+| `{version_suffix_dash}` | Suffix with dash |
+| `{profile}` | Profile name (debug/release) |
+| `{name}` | Package name |