feat: switch/case statement

- Add  syntax
- Desugars to if-else chain in HIR lowering (both compilers)
- No new HIR/C backend nodes needed — reuses hIf/hBinary/hBlock
- Supports integer, char, and enum tag dispatch
- Case body can be single statement (no braces required)

bootstrap/ast.nim

@@ -239,6 +239,7 @@ type
     skComptime
     skEmit
     skDefer
+    skSwitch
     skDecl
 
   ElseIf* = object
@@ -246,6 +247,11 @@ type
     cond*: Expr
     blk*: Block
 
+  SwitchCase* = object
+    loc*: SourceLocation
+    caseValue*: Expr
+    caseBody*: Block
+
   Block* = ref object
     loc*: SourceLocation
     stmts*: seq[Stmt]
@@ -301,6 +307,10 @@ type
       stmtEmitEvaluated*: string  ## filled by sema CTFE
     of skDefer:
       stmtDeferBody*: Expr
+    of skSwitch:
+      stmtSwitchExpr*: Expr
+      stmtSwitchCases*: seq[SwitchCase]
+      stmtSwitchDefault*: Block
     of skDecl:
       stmtDecl*: Decl