cosmic-conf: parser, schema registry, resolve with projection folding

Spike results corrected the spec: ThemeBuilder.gaps is (outer, inner) at
cosmic-theme/src/model/theme.rs:895, lives under CosmicTheme.{Dark,Light}.Builder
rather than CosmicTk, and fans out to two components. Entry therefore carries
targets: &[Target].

34 tests, including the folding property that keeps gaps_out from clobbering
gaps_in. Bare #rrggbb colours rejected: # begins a comment, as in Hyprland.

.gitignore

@@ -2,5 +2,6 @@
 .omc/
 
 # Rust
-/target/
+target/
 **/*.rs.bk
+vendor/