feat(lsp): add completions for type system keywords

Added top-level keyword completions for:
- concept: with semicolon-terminated snippet
- sub_concept: with dot notation snippet
- concept_comparison: with brace-delimited snippet

Added test verifying type system keywords appear in
completions with correct snippet formatting.

src/lsp/completion.rs

@@ -683,6 +683,9 @@ fn top_level_keyword_completions() -> Vec<CompletionItem> {
         keyword_item("location", "Define a location", "location ${1:Name} {\n    $0\n}"),
         keyword_item("species", "Define a species", "species ${1:Name} {\n    $0\n}"),
         keyword_item("use", "Import declarations", "use ${1:path::to::item};"),
+        keyword_item("concept", "Define an algebraic data type", "concept ${1:Name};"),
+        keyword_item("sub_concept", "Define a sub-type", "sub_concept ${1:Parent}.${2:Name} {\n    $0\n}"),
+        keyword_item("concept_comparison", "Pattern match on concepts", "concept_comparison ${1:Name} {\n    $0\n}"),
     ]
 }