Add double and float support to scan test

- Add double_type and float_type to scan_type enum
- Add double* and float* pointers to scan_arg union
- Add constructors for double and float scan arguments
- Add switch cases for double and float types in visit()
- Implement basic read() functions for floating-point parsing

This partially resolves the TODO comment 'more types' in scan.h by adding
support for the two most commonly needed floating-point types.

Author: Sahil Sinha

test/scan.h

@@ -229,6 +229,8 @@ enum class scan_type {
   uint_type,
   long_long_type,
   ulong_long_type,
+  double_type,
+  float_type,
   string_type,
   string_view_type,
   custom_type
@@ -251,6 +253,8 @@ template <typename Context> class basic_scan_arg {
     unsigned* uint_value_;
     long long* long_long_value_;
     unsigned long long* ulong_long_value_;
+    double* double_value_;
+    float* float_value_;
     std::string* string_;
     string_view* string_view_;
     detail::custom_scan_arg<Context> custom_;
@@ -276,6 +280,10 @@ template <typename Context> class basic_scan_arg {
       : type_(scan_type::long_long_type), long_long_value_(&value) {}
   FMT_CONSTEXPR basic_scan_arg(unsigned long long& value)
       : type_(scan_type::ulong_long_type), ulong_long_value_(&value) {}
+  FMT_CONSTEXPR basic_scan_arg(double& value)
+      : type_(scan_type::double_type), double_value_(&value) {}
+  FMT_CONSTEXPR basic_scan_arg(float& value)
+      : type_(scan_type::float_type), float_value_(&value) {}
   FMT_CONSTEXPR basic_scan_arg(std::string& value)
       : type_(scan_type::string_type), string_(&value) {}
   FMT_CONSTEXPR basic_scan_arg(string_view& value)
@@ -305,6 +313,10 @@ template <typename Context> class basic_scan_arg {
       return vis(*long_long_value_);
     case scan_type::ulong_long_type:
       return vis(*ulong_long_value_);
+    case scan_type::double_type:
+      return vis(*double_value_);
+    case scan_type::float_type:
+      return vis(*float_value_);
     case scan_type::string_type:
       return vis(*string_);
     case scan_type::string_view_type:
@@ -457,6 +469,47 @@ auto read(scan_iterator it, T& value, const format_specs& specs = {})
   return it;
 }
 
+auto read(scan_iterator it, double& value, const format_specs& = {})
+    -> scan_iterator {
+  if (it == scan_sentinel()) return it;
+  
+  // Simple floating-point parsing
+  bool negative = *it == '-';
+  if (negative) {
+    ++it;
+    if (it == scan_sentinel()) report_error("invalid input");
+  }
+  
+  double result = 0.0;
+  // Parse integer part
+  while (it != scan_sentinel() && *it >= '0' && *it <= '9') {
+    result = result * 10.0 + (*it - '0');
+    ++it;
+  }
+  
+  // Parse decimal part if present
+  if (it != scan_sentinel() && *it == '.') {
+    ++it;
+    double fraction = 0.1;
+    while (it != scan_sentinel() && *it >= '0' && *it <= '9') {
+      result += (*it - '0') * fraction;
+      fraction *= 0.1;
+      ++it;
+    }
+  }
+  
+  value = negative ? -result : result;
+  return it;
+}
+
+auto read(scan_iterator it, float& value, const format_specs& specs = {})
+    -> scan_iterator {
+  double temp;
+  it = read(it, temp, specs);
+  value = static_cast<float>(temp);
+  return it;
+}
+
 auto read(scan_iterator it, std::string& value, const format_specs& = {})
     -> scan_iterator {
   while (it != scan_sentinel() && *it != ' ') value.push_back(*it++);