feat: generic enums, fix is-operator, Type_Eq, hardcoded limit diagnostics

- feat: generic enum support (parser, lowering, codegen — both selfhost + bootstrap)
  - enum Result<T,E> { Ok(T), Err(E) } parsing + monomorphization
  - tag constant mangling in monomorphized function bodies
  - data field access (l-value + r-value) for generated enum instances
  - multiple concrete instances in same file
  - HIR walker for enum reference mangling (selfhost + bootstrap)

- feat: stdlib Result<T,E> and Option<T> made truly generic
  - breaking: explicit type args required (Result<int, String>)

- fix: 'is' operator — lowering to hBinary tag comparison + C backend fallback
- fix: Type_Eq structural comparison (inner types for pointer/slice/tuple)
- fix: hardcoded limit diagnostics (>8 params/variants/captures now emit errors)
- docs: Iter<T> safety warning for dangling pointer
- docs: IMPROVEMENTS.md — comprehensive plan and changelog
- test: generic_enum example added to EXAMPLES

All tests pass (0 FAIL). Selfhost loop deterministic.

bootstrap/parser.nim

@@ -1404,6 +1404,7 @@ proc parseEnumDecl(p: var Parser, isPublic: bool): Decl =
   let loc = p.currentLoc
   discard p.expect(tkEnum, "expected 'enum'")
   let name = p.expect(tkIdent, "expected enum name").text
+  let typeParams = p.parseTypeParams()
   var baseType: TypeExpr = nil
   if p.check(tkColon):
     discard p.advance()
@@ -1444,6 +1445,7 @@ proc parseEnumDecl(p: var Parser, isPublic: bool): Decl =
   discard p.expect(tkRBrace, "expected '}' to close enum")
   return Decl(kind: dkEnum, loc: loc, isPublic: isPublic,
               declEnumName: name, declEnumBaseType: baseType,
+              declEnumTypeParams: typeParams,
               declEnumVariants: variants)
 
 proc parseUnionDecl(p: var Parser, isPublic: bool): Decl =