Bond graph ports and drawing added

BEdit/AGENTS.md

@@ -59,7 +59,12 @@ Keep the root of `src/bedit` minimal. New UI modules should go into the relevant
 - Port orientation is presented as one unified list in the UI, while the model
   indexes inputs and outputs separately for connection semantics.
 - Port types are registered in `core/port_types.py`. Only compatible types may be
-  connected. `signal` is currently the only type.
+  connected. Signal ports connect by type; power ports additionally require the
+  same domain. Editable power domains and causalities live in
+  `core/power_domains.py`.
+- Power ports may use `indifferent` orientation and can act as either connection
+  endpoint. For two indifferent ports, click order determines source/arrow
+  direction; otherwise output/input semantics determine direction.
 - Port connector type and signal value type are separate. Signal ports and
   parameters carry editable value type, quantity, unit, row/column dimensions,
   and description metadata. Editable quantity/unit suggestions live in
@@ -78,7 +83,8 @@ Keep the root of `src/bedit` minimal. New UI modules should go into the relevant
   intersection of the owning hitbox and its center-to-adjacent-route-point ray.
 - Connection appearance is configured per port type in
   `gui/graphics/connection_styles.py`, including color, width, pen style, and
-  source/target arrowheads. Do not scatter those constants through painters.
+  source/target arrowheads. Arrow styles are `open`, `half`, or `filled`. Do not
+  scatter those constants through painters.
 - Graph annotations are `box`, `line`, or `text` objects in `Graph.annotations`.
   They use integer layers below or above graph layer 0. Annotation lines reuse
   connection polyline and absolute `properties.waypoints` semantics. Graph