feat: Export entrypoint metadata in Python and fix bug in import (#2434)

This PR ensures that the `core.entrypoint` metadata is exported in the
Python bindings. It also fixes a bug in `hugr-core` import that added
children of the root node to the entrypoint instead of the root.

Author: zrho

hugr-core/src/import.rs

@@ -30,8 +30,8 @@ use crate::{
     },
 };
 use fxhash::FxHashMap;
-use hugr_model::v0 as model;
 use hugr_model::v0::table;
+use hugr_model::v0::{self as model};
 use itertools::{Either, Itertools};
 use smol_str::{SmolStr, ToSmolStr};
 use thiserror::Error;
@@ -433,7 +433,7 @@ impl<'a> Context<'a> {
         let region_data = self.get_region(self.module.root)?;
 
         for node in region_data.children {
-            self.import_node(*node, self.hugr.entrypoint())?;
+            self.import_node(*node, self.hugr.module_root())?;
         }
 
         for meta_item in region_data.meta {

hugr-py/src/hugr/model/export.py

@@ -49,8 +49,6 @@ def __init__(self, hugr: Hugr):
         self.link_names: dict[InPort | OutPort, str] = {}
         self.link_next = 0
 
-        # TODO: Store the hugr entrypoint
-
         for a, b in self.hugr.links():
             self.link_ports.union(a, b)
 
@@ -397,10 +395,18 @@ def export_json_meta(self, node: Node) -> list[model.Term]:
 
         return meta
 
+    def export_entrypoint_meta(self, node: Node) -> list[model.Term]:
+        """Export entrypoint metadata if the node is the module entrypoint."""
+        if self.hugr.entrypoint == node:
+            return [model.Apply("core.entrypoint")]
+        else:
+            return []
+
     def export_region_module(self, node: Node) -> model.Region:
         """Export a module node as a module region."""
         node_data = self.hugr[node]
         meta = self.export_json_meta(node)
+        meta += self.export_entrypoint_meta(node)
         children = []
 
         for child in node_data.children:
@@ -419,7 +425,8 @@ def export_region_dfg(self, node: Node) -> model.Region:
         target_types: model.Term = model.Wildcard()
         sources = []
         targets = []
-        meta = []
+
+        meta = self.export_entrypoint_meta(node)
 
         for child in node_data.children:
             child_data = self.hugr[child]
@@ -489,6 +496,7 @@ def export_region_cfg(self, node: Node) -> model.Region:
         source_types: model.Term = model.Wildcard()
         target_types: model.Term = model.Wildcard()
         children = []
+        meta = self.export_entrypoint_meta(node)
 
         for child in node_data.children:
             child_data = self.hugr[child]
@@ -540,6 +548,7 @@ def export_region_cfg(self, node: Node) -> model.Region:
             sources=[source],
             signature=signature,
             children=children,
+            meta=meta,
         )
 
     def export_symbol(