feat: add pkg-config system dependencies and fix dcr new in non-ascii paths

docs/reference/dcr-toml.mdx

@@ -111,7 +111,11 @@ Never fails the command — warnings only. Pre-release suffixes (`1.0.0-dev`) ar
 | `qt` | `false` | Enable automatic Qt meta-object handling (MOC, UIC, RCC) |
 
 Settings from raw config (not in typed struct):
-- `pkg_config` — list of pkg-config packages
+- `pkg_config` — list of packages queried for build-wide compiler/linker flags
+
+Dependencies can also declare a system package directly, for example
+`fmt = { pkg-config = "fmt" }`. This records the package in `dcr.lock` and applies
+the flags returned by `pkg-config --cflags` and `pkg-config --libs`.
 - `ldscript` — linker script path
 - `build.steps` / `build.post_steps` — codegen steps
 

docs/reference/dependencies.mdx

@@ -6,7 +6,7 @@ sidebar_label: Dependencies
 
 ## Formats
 
-The `[dependencies]` section supports three formats:
+The `[dependencies]` section supports registry, path, Git, and `pkg-config` dependencies:
 
 ### String (registry)
 
@@ -44,6 +44,22 @@ names (`libs`):
 dcr-math = { path = "../deps/dcr-math", include = ["include"], lib = ["lib"], libs = ["dcr-math"] }
 ```
 
+### Table (`pkg-config`)
+
+Use an installed `.pc` package when the library is provided by the operating system
+or another system package manager. DCR invokes `pkg-config --cflags` and
+`pkg-config --libs`, preserving include, library, linker, and other flags:
+
+```toml
+[dependencies]
+fmt = { pkg-config = "fmt" }
+openssl = { pkg-config = ["openssl", "libcrypto"] }
+```
+
+The `pkg-config` executable and the named packages must be available in `PATH` and
+`PKG_CONFIG_PATH`. These dependencies are not downloaded or built by DCR; they are
+recorded in `dcr.lock` with a `pkg-config+...` source.
+
 ## Registry
 
 DCR uses a package registry for dependency lookup by name.