feat(uci): derive UCI schema from settings

Add uci-derive proc-macro crate with UciConfig and UciSection derive
macros that generate UCI schema metadata from Settings structs. The
schema maps UCI section types and option names to config-rs field
paths. UciSource::from_schema() creates a source from the derived
schema, mapping known UCI options to their Settings fields while
preserving unknown options under raw UCI-derived paths.

Cargo.toml

@@ -29,6 +29,7 @@ flume = { version = "0.12.0",  default-features = false, features = ["eventual-f
 config = { version = "0.15.25", default-features = false }
 csv = "1.4.0"
 serde = { version = "1", features = ["derive"] }
+uci-derive = { path = "crates/uci-derive" }
 
 [dependencies.log]
 features = ["std"]
@@ -46,3 +47,7 @@ default = ["toml", "uci-vendored"]
 toml = ["config/toml"]
 uci = ["dep:rust-uci"]
 uci-vendored = ["uci", "rust-uci/vendored"]
+
+[workspace]
+members = ["crates/uci-derive"]
+resolver = "3"