Language: enum declarations

Adds \`enum Name { V1, V2, ... }\` as a top-level declaration. Each
variant is registered as a \`u8\` constant equal to its index in the
declaration. Variant names are global and must be unique across all
enums and other symbols (E0501 on collision).

- Lexer: \`enum\` keyword
- AST: \`EnumDecl { name, variants, span }\` field on \`Program\`
- Parser: top-level \`enum\` syntax with optional trailing commas
- Analyzer: \`register_enum\` flattens variants into the symbol table
- IR lowering and AST codegen: variants resolve through the existing
  \`const_values\` path
- Tests cover parsing, duplicate detection, and usage

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/ir/lowering.rs

@@ -126,6 +126,13 @@ impl LoweringContext {
             }
         }
 
+        // Register enum variants as constants (0, 1, 2, ... per enum)
+        for e in &program.enums {
+            for (i, (variant, _)) in e.variants.iter().enumerate() {
+                self.const_values.insert(variant.clone(), i as u16);
+            }
+        }
+
         // Lower globals
         for var in &program.globals {
             let var_id = self.get_or_create_var(&var.name);