Adding "engine" project for plantuml

Small project example:
```
startuml{project}

Project starts 2026-03-17

saturday are closed
sunday are closed

' Milestones
[Review 01] happens at 2026-03-30
[Review 02] happens on 21 days after [Review 01]'s end
@enduml
```

Author: albert-github

addon/doxmlparser/doxmlparser/compound.py

@@ -1131,6 +1131,7 @@ class DoxPlantumlEngine(str, Enum):
     REGEX='regex'
     EBNF='ebnf'
     FILES='files'
+    PROJECT='project'
 
 
 class DoxProtectionKind(str, Enum):
@@ -24555,7 +24556,7 @@ def validate_DoxPlantumlEngine(self, value):
                 self.gds_collector_.add_message('Value "%(value)s"%(lineno)s is not of the correct base simple type (str)' % {"value": value, "lineno": lineno, })
                 return False
             value = value
-            enumerations = ['uml', 'bpm', 'wire', 'dot', 'ditaa', 'salt', 'math', 'latex', 'gantt', 'mindmap', 'wbs', 'yaml', 'creole', 'json', 'flow', 'board', 'git', 'hcl', 'regex', 'ebnf', 'files']
+            enumerations = ['uml', 'bpm', 'wire', 'dot', 'ditaa', 'salt', 'math', 'latex', 'gantt', 'mindmap', 'wbs', 'yaml', 'creole', 'json', 'flow', 'board', 'git', 'hcl', 'regex', 'ebnf', 'files', 'project']
             if value not in enumerations:
                 lineno = self.gds_get_node_lineno_()
                 self.gds_collector_.add_message('Value "%(value)s"%(lineno)s does not match xsd enumeration restriction on DoxPlantumlEngine' % {"value" : encode_str_2_3(value), "lineno": lineno} )

doc/commands.dox

@@ -3590,7 +3590,7 @@ class Receiver
   PlantUML `@start...` command. This will look like `@start<engine>` where currently supported are
   the following `<engine>`s: `uml`, `bpm`, `wire`, `dot`, `ditaa`, `salt`, `math`, `latex`,
   `gantt`, `mindmap`, `wbs`, `yaml`, `creole`, `json`, `flow`, `board`, `git`, `hcl`, `regex`, `ebnf`,
-  `files`, `chen` and `chronology`.
+  `project`, `files`, `chen` and `chronology`.
   By default the `<engine>` is `uml`. The `<engine>` can be specified as an option.
   Also the file to write the resulting image to can be specified by means of an option, see the
   description of the first (optional) argument for details.

src/docnode.cpp

@@ -72,7 +72,7 @@ static const StringUnorderedSet g_plantumlEngine {
   "salt", "math", "latex", "gantt", "mindmap",
   "wbs", "yaml", "creole", "json", "flow",
   "board", "git", "hcl", "regex", "ebnf",
-  "files", "chen", "chronology"
+  "project", "files", "chen", "chronology"
 };
 
 //---------------------------------------------------------------------------

templates/xml/compound.xsd

@@ -1209,6 +1209,7 @@
       <xsd:enumeration value="regex"/>
       <xsd:enumeration value="ebnf"/>
       <xsd:enumeration value="files"/>
+      <xsd:enumeration value="project"/>
     </xsd:restriction>
   </xsd:simpleType>