Unified: Add Swift corpus tests

Add corpus test cases for Swift covering closures, collections, control
flow, functions, literals, loops, operators, optionals/errors, types,
and variables. Update existing desugar.txt with raw parse sections.

Note: operator nodes currently render their node ID instead of the actual
operator text (e.g. operator "3" instead of operator "+"). This will be
fixed in the next commit.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: asgerf

unified/extractor/tests/corpus/swift/closures.txt

@@ -0,0 +1,188 @@
+===
+Closure with explicit parameters
+===
+
+let f = { (x: Int) -> Int in x * 2 }
+
+---
+
+source_file
+  property_declaration
+    name:
+      pattern
+        bound_identifier: simple_identifier "f"
+    value:
+      lambda_literal
+        type:
+          lambda_function_type
+            name:
+              user_type
+                type_identifier "Int"
+            lambda_function_type_parameters
+              lambda_parameter
+                name:
+                  simple_identifier "x"
+                  user_type
+                    type_identifier "Int"
+        statements
+          multiplicative_expression
+            lhs: simple_identifier "x"
+            op: *
+            rhs: integer_literal "2"
+    value_binding_pattern
+      mutability: let
+
+---
+
+top_level
+  body: unsupported_node "let f = { (x: Int) -> Int in x * 2 }"
+
+===
+Closure with shorthand parameters
+===
+
+let f = { $0 + $1 }
+
+---
+
+source_file
+  property_declaration
+    name:
+      pattern
+        bound_identifier: simple_identifier "f"
+    value:
+      lambda_literal
+        statements
+          additive_expression
+            lhs: simple_identifier "$0"
+            op: +
+            rhs: simple_identifier "$1"
+    value_binding_pattern
+      mutability: let
+
+---
+
+top_level
+  body: unsupported_node "let f = { $0 + $1 }"
+
+===
+Trailing closure
+===
+
+xs.map { $0 * 2 }
+
+---
+
+source_file
+  call_expression
+    navigation_expression
+      suffix:
+        navigation_suffix
+          suffix: simple_identifier "map"
+      target: simple_identifier "xs"
+    call_suffix
+      lambda_literal
+        statements
+          multiplicative_expression
+            lhs: simple_identifier "$0"
+            op: *
+            rhs: integer_literal "2"
+
+---
+
+top_level
+  body: unsupported_node "xs.map { $0 * 2 }"
+
+===
+Closure with capture list
+===
+
+let f = { [weak self] in self?.doThing() }
+
+---
+
+source_file
+  property_declaration
+    name:
+      pattern
+        bound_identifier: simple_identifier "f"
+    value:
+      lambda_literal
+        captures:
+          capture_list
+            capture_list_item
+              name: simple_identifier "self"
+              ownership_modifier
+        statements
+          call_expression
+            navigation_expression
+              suffix:
+                navigation_suffix
+                  suffix: simple_identifier "doThing"
+              target:
+                self_expression
+                ?
+            call_suffix
+              value_arguments
+    value_binding_pattern
+      mutability: let
+
+---
+
+top_level
+  body: unsupported_node "let f = { [weak self] in self?.doThing() }"
+
+===
+Multi-statement closure
+===
+
+let f = { (x: Int) -> Int in
+  let y = x + 1
+  return y * 2
+}
+
+---
+
+source_file
+  property_declaration
+    name:
+      pattern
+        bound_identifier: simple_identifier "f"
+    value:
+      lambda_literal
+        type:
+          lambda_function_type
+            name:
+              user_type
+                type_identifier "Int"
+            lambda_function_type_parameters
+              lambda_parameter
+                name:
+                  simple_identifier "x"
+                  user_type
+                    type_identifier "Int"
+        statements
+          property_declaration
+            name:
+              pattern
+                bound_identifier: simple_identifier "y"
+            value:
+              additive_expression
+                lhs: simple_identifier "x"
+                op: +
+                rhs: integer_literal "1"
+            value_binding_pattern
+              mutability: let
+          control_transfer_statement
+            result:
+              multiplicative_expression
+                lhs: simple_identifier "y"
+                op: *
+                rhs: integer_literal "2"
+    value_binding_pattern
+      mutability: let
+
+---
+
+top_level
+  body: unsupported_node "let f = { (x: Int) -> Int in\n  let y = x + 1\n  return y * 2\n}"