Unified: add tuple_pattern and sequence_condition; refine if-let/guard mapping

ast_types.yml additions:
- tuple_pattern { element*: pattern } in the pattern supertype.
- sequence_condition { stmt*: stmt, condition: condition } in the
  condition supertype.

swift.rs:
- Map Swift tuple destructuring (e.g. `let (a, b) = pair`) to the new
  tuple_pattern instead of synthesizing an apply_pattern.
- if-let / guard-let: explicitly match the value_binding_pattern
  (the `let` keyword) and bind the source expression as the next
  condition child, so `let` no longer leaks into the output.

unified/extractor/ast_types.yml

@@ -18,6 +18,7 @@ supertypes:
   condition:
     - expr_condition
     - let_pattern_condition
+    - sequence_condition
     - unsupported_node
   pattern:
     - var_pattern
@@ -92,6 +93,13 @@ named:
   expr_condition:
     expr: expr
 
+  # A series of statements that are executed before evaluating the trailing condition.
+  # Useful for languages where a conditional clause may be preceded by side-effecting
+  # syntactic elements (e.g. binding clauses) that don't themselves form a condition.
+  sequence_condition:
+    stmt*: stmt
+    condition: condition
+
   # Evaluate 'expr' and match its result against 'pattern', and return true if it matches.
   # Variables bound by the pattern will be in scope within the 'true' branch controlled by this condition.
   let_pattern_condition:

unified/extractor/src/languages/swift/swift.rs

@@ -157,29 +157,33 @@ fn translation_rules() -> Vec<yeast::Rule> {
         ),
         // ---- Guard statement ----
         // `guard let x = e else { ... }` — currently only handles the
-        // let-binding form, since plain `guard cond else { ... }` is not
-        // exercised by any existing corpus test.
+        // let-binding form. The Swift parser models the `let` keyword as a
+        // `value_binding_pattern` child of `condition`, followed by an
+        // unnamed `=` and the source expression.
         rule!(
             (guard_statement
                 bound_identifier: (simple_identifier) @id
-                condition: (_)* @cond_children
+                condition: (value_binding_pattern)
+                condition: (_) @value
                 (else)
                 (statements) @else_branch)
             =>
             (guard_if_stmt
                 condition: (let_pattern_condition
                     pattern: (var_pattern identifier: (identifier #{id}))
-                    value: {*cond_children.last().unwrap()})
+                    value: {value})
                 else: {else_branch})
         ),
         // ---- If statement ----
         // if-let binding (with optional else branch). The Swift parser puts
-        // the bound name in `bound_identifier` and the source expression as
-        // the last child of the multi-child `condition` field.
+        // the bound name in `bound_identifier`, the `let` keyword as a
+        // `value_binding_pattern` child of `condition`, and the source
+        // expression as a separate child of `condition`.
         rule!(
             (if_statement
                 bound_identifier: (simple_identifier) @id
-                condition: (_)* @cond_children
+                condition: (value_binding_pattern)
+                condition: (_) @value
                 (statements) @then
                 (else)
                 (_) @else_branch)
@@ -187,20 +191,21 @@ fn translation_rules() -> Vec<yeast::Rule> {
             (if_stmt
                 condition: (let_pattern_condition
                     pattern: (var_pattern identifier: (identifier #{id}))
-                    value: {*cond_children.last().unwrap()})
+                    value: {value})
                 then: {then}
                 else: {else_branch})
         ),
         rule!(
             (if_statement
                 bound_identifier: (simple_identifier) @id
-                condition: (_)* @cond_children
+                condition: (value_binding_pattern)
+                condition: (_) @value
                 (statements) @then)
             =>
             (if_stmt
                 condition: (let_pattern_condition
                     pattern: (var_pattern identifier: (identifier #{id}))
-                    value: {*cond_children.last().unwrap()})
+                    value: {value})
                 then: {then})
         ),
         // With explicit else branch (block or chained if).