yoda: cosmic-theme cleanup (4→0 warnings) — workspace at 0 warnings total

theme::Builder::build:
- #[allow(unused_assignments)] with a note explaining the pattern: the
  component_hovered/pressed_overlay seeds at the top of the function
  are overwritten by every container block (primary, secondary, …)
  before being read, which is what unused_assignments flagged.

vs_code.rs:
- Add doc + # Errors section to Theme::apply_vs_code and the
  associated Theme::reset_vs_code (was missing under #![warn(missing_docs)]).

Cargo.toml profile move:
- cosmic-theme had a [profile.dev.package] block that Cargo silently
  ignores in non-root manifests. Move the insta/similar opt-level=3
  hint to the workspace root and drop the dead block from the
  cosmic-theme manifest. Removes the cargo-level "profiles for the
  non root package will be ignored" notice that came up on every check.

Leyoda 2026 – GPLv3

Cargo.toml

@@ -264,5 +264,13 @@ exclude = ["iced", "examples"]
 [workspace.dependencies]
 dirs = "6.0.0"
 
+# Speed up snapshot diffing in cosmic-theme tests. Cargo silently ignores
+# [profile.*] blocks in non-root manifests, so this lives at the
+# workspace root.
+[profile.dev.package.insta]
+opt-level = 3
+[profile.dev.package.similar]
+opt-level = 3
+
 [dev-dependencies]
 tempfile = "3.27.0"