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.

src/settings/uci_schema.rs

@@ -0,0 +1,29 @@
+// SPDX-FileCopyrightText: 2026-Present Nils Andreas Svee mailto:contact@lochnair.net (github @Lochnair)
+//
+// SPDX-License-Identifier: MPL-2.0
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub(crate) struct UciOptionMapping {
+    pub(crate) uci_option: &'static str,
+    pub(crate) config_field: &'static str,
+}
+
+#[derive(Clone, Copy, Debug)]
+pub(crate) struct UciSectionMapping {
+    /// UCI section type, corresponding to `@section_type[0]`.
+    pub(crate) uci_section: &'static str,
+
+    /// Top-level config-rs / Serde settings key.
+    pub(crate) config_section: &'static str,
+
+    pub(crate) options: &'static [UciOptionMapping],
+}
+
+pub(crate) trait UciSectionSchema {
+    const UCI_OPTIONS: &'static [UciOptionMapping];
+}
+
+pub(crate) trait UciConfigSchema {
+    const UCI_PACKAGE: &'static str;
+    const UCI_SECTIONS: &'static [UciSectionMapping];
+}