refactor: modularize build core engine, update workspaces, cache, and test suite

docs/reference/dcr-toml.mdx

@@ -41,7 +41,7 @@ Main project configuration file. Located at the project root.
 |-------|----------|-------------|
 | `name` | yes | Project name |
 | `version` | yes | Semantic version |
-| `type` | yes | `app`, `lib`, `none` |
+| `type` | no | `app`, `lib`, `none` (defaults to `"none"`) |
 | `license` | no | SPDX license identifier |
 | `author` | no | Author |
 
@@ -62,10 +62,10 @@ author = "John Doe"
 
 | Field | Default | Description |
 |-------|---------|-------------|
-| `language` | `"c"` | `"c"`, `"c++"`, `"cpp"`, `"cxx"`, `"asm"` |
+| `language` | `"c"` | `"c"`, `"c++"`, `"cpp"`, `"cxx"`, `"asm"` (optional, defaults to `"c"`) |
 | `standard` | `"c11"` | C standard (`c11`, `c17`, `c23`) |
-| `cxx_standard` | `"c++20"` | C++ standard (`c++17`, `c++20`, `c++23`) |
-| `compiler` | `"clang"` | Preferred compiler |
+| `cxx_standard` | — | C++ standard (`c++17`, `c++20`, `c++23`) |
+| `compiler` | `"clang"` | Preferred compiler (optional, defaults to `"clang"`) |
 | `kind` | `"bin"` | `bin`, `staticlib`, `sharedlib`, `efi`, `elf`, `none`, `custom` |
 | `target` | host | Target triple for cross-compilation |
 | `platform` | `"native"` | `native`, `efi` |
@@ -80,13 +80,13 @@ author = "John Doe"
 | `inherit` | `false` | Inherit build from workspace root |
 | `clean` | `[]` | Glob patterns for custom clean paths |
 | `out_dir` | `""` | Custom output directory |
-| `workspace_only` | `false` | Workspace-only, not built standalone |
+| `workspace_only` | `false` | Workspace-only, not built standalone (no `language`/`compiler` required) |
 | `freestanding` | `false` | Compile in freestanding mode (`-ffreestanding` + `-nostdlib -static`) |
-| `opt_level` | `"0"` (debug) / `"3"` (release) | Optimization level: `0`-`3`, `"s"`, `"z"` |
-| `debug` | `true` (debug) / `false` (release) | Emit debug symbols (`-g`) |
+| `opt_level` | — | Optimization level: `0`-`3`, `"s"`, `"z"` (derived from profile if omitted) |
+| `debug` | profile-based | Emit debug symbols (`-g`): `true` in debug, `false` in release |
 | `lto` | `false` | Link-time optimization (`-flto` for both compiler and linker) |
 | `strip` | `false` | Strip symbols from output (`-s` in ldflags) |
-| `warnings` | `["all", "extra"]` (debug) / `[]` (release) | Warning flags (e.g. `"all"`, `"extra"`, `"pedantic"`) |
+| `warnings` | `[]` | Warning flags (e.g. `"all"`, `"extra"`, `"pedantic"`); engine adds `-Wall -Wextra` in debug if empty |
 | `panic` | `""` | Panic strategy: `"abort"` disables exceptions and unwind tables |
 | `codegen-units` | `""` | Max parallel compilation jobs (`"0"` = auto) |
 | `qt` | `false` | Enable automatic Qt meta-object handling (MOC, UIC, RCC) |
@@ -96,6 +96,30 @@ Settings from raw config (not in typed struct):
 - `ldscript` — linker script path
 - `build.steps` / `build.post_steps` — codegen steps
 
+## Per-language overrides: `[build.c]`, `[build.cxx]`, `[build.asm]`
+
+Each language can have its own table that overrides the flat `[build]` settings:
+
+```toml
+[build]
+compiler = "clang"
+standard = "c11"
+
+[build.c]
+standard = "c23"
+compiler = "gcc"
+
+[build.cxx]
+standard = "c++23"
+compiler = "g++"
+
+[build.asm]
+compiler = "nasm"
+flags = ["-felf64"]
+```
+
+The flat `[build]` acts as fallback; per-language tables take precedence for their language.
+
 Example:
 
 ```toml

docs/reference/dependencies.mdx

@@ -41,24 +41,22 @@ DCR uses a package registry for dependency lookup by name.
 
 ```toml
 # ~/.dcr/config.toml
-[[registry]]
-name = "default"
+[registry.default]
 url = "https://index.dcr.pm"
+priority = 1
 ```
 
 Registry priority: order in `config.toml`. The `DCR_INDEX_PATH` variable overrides the path to `index.json`.
 
 ## Git dependencies
 
-DCR clones git repositories via `git2`:
+Git dependencies are parsed and recorded in `dcr.lock`. DCR supports specifying branch/tag/rev for git sources:
 
 - `branch` — switch to a branch
 - `tag` — switch to a tag
 - `rev` — switch to a specific commit
 - `features` — feature flags (parsed, but does not affect build)
 
-Cache path: `~/.dcr/git/<hash>/`.
-
 ## Path dependencies
 
 Local paths. DCR automatically discovers include and lib directories from the neighbor's `dcr.toml`.

docs/reference/workspaces.mdx

@@ -65,3 +65,21 @@ standard = "c17"
 
 # member inherits language and standard
 ```
+
+## Workspace-only root (no build of its own)
+
+A workspace root can set `workspace_only = true` — it won't be built itself, and doesn't need `language` or `compiler`:
+
+```toml
+[package]
+name = "my-workspace"
+version = "0.1.0"
+
+[build]
+workspace_only = true
+kind = "bin"
+
+[workspace]
+lib-core = { path = "lib-core" }
+app = { path = "app", deps = ["lib-core"] }
+```