feat: defer statement support in both compilers

- Add  statement for function-level deferred execution
- Deferred expressions run in LIFO order on function exit (return or implicit)
- Bootstrap: desugar defers in hir_lower.nim (inject before return + end of func)
- Selfhost: emit defers in c_backend.bux via CEmitter defer stack
- Both: add tkDefer token, skDefer AST node, hDefer HIR node
- Parser, lexer, sema, token files updated in both bootstrap/ and src/

bootstrap/parser.nim

@@ -959,6 +959,12 @@ proc parseStmt(p: var Parser): Stmt =
       # No-op: emit literal 0 as expression statement
       let zeroTok = Token(kind: tkIntLiteral, text: "0", loc: loc)
       return Stmt(kind: skExpr, loc: loc, stmtExpr: Expr(kind: ekLiteral, loc: loc, exprLit: zeroTok))
+  of tkDefer:
+    discard p.advance()
+    let expr = p.parseExpr()
+    if p.check(tkSemicolon):
+      discard p.advance()
+    return Stmt(kind: skDefer, loc: loc, stmtDeferBody: expr)
   of tkFunc, tkStruct, tkEnum, tkUnion, tkInterface, tkExtend, tkModule,
      tkImport, tkConst, tkType, tkExtern, tkPub:
     # Local declaration