feat(config): add config-rs settings support

Introduce config-rs and Serde support with a nested Settings model.
Add config and serde dependencies with toml feature for file-based
configuration loading. The Settings struct provides typed configuration
with sensible defaults for network, output, observability, and advanced
settings sections. Environment variable override via SQMA__ prefix is
supported. The legacy Config remains in use; migration follows later.

Cargo.toml

@@ -26,6 +26,8 @@ rustix = { version = "1.1", features = ["fs", "system", "thread", "time"] }
 thiserror = "2.0"
 libc = "0.2"
 flume = { version = "0.12.0",  default-features = false, features = ["eventual-fairness", "select"] }
+config = { version = "0.15.25", default-features = false }
+serde = { version = "1", features = ["derive"] }
 
 [dependencies.log]
 features = ["std"]
@@ -37,4 +39,6 @@ netlink-socket2 = { version = "0.3.3" }
 rust-uci = { git = "https://github.com/namib-project/rust-uci.git", rev = "3e7b6153b6df92b45428c32bb6461756563deecb", optional = true }
 
 [features]
+default = ["toml"]
+toml = ["config/toml"]
 uci = ["dep:rust-uci"]