feat(diagram): compact cells, position and size as attributes on the mxCell

The model now writes a shape as one self-closing mxCell with x, y, w and h
and an edge as one with source and target; compact-cells.ts fills in
vertex, edge, parent="1" and the mxGeometry element after validation. The
long form stays accepted, so waypoints, edge labels and anything else keep
their mxGeometry. The diagram shown to the model (the system context, the
edit error texts and the MCP get_diagram result) is folded into the same
notation, so the model reads what it is asked to write; the canvas and the
saved file stay standard draw.io XML. Folding then expanding the 652 valid
production diagrams gives the same cells back for every one of them.

The auto-fix gains one step for the slip this notation invites: a missing
closing quote before the self-closing slash (h="112/>).

Prompts and the shared examples use the compact form; the single waypoint
example keeps the long form on purpose.

Measured on the five start-screen examples, same prompt, against the
named-styles build: gpt-6-luna 19% fewer output tokens, Opus 5.5 26% fewer
(35% and 41% against main). Every shape was written compactly on the first
try; edits used it too and all applied.

lib/system-prompts.ts

@@ -101,25 +101,23 @@ Note that:
 
 When using edit_diagram tool:
 - Use operations: update (modify cell by id), add (new cell), delete (remove cell by id)
-- For update/add: provide cell_id and complete new_xml (full mxCell element including mxGeometry)
+- For update/add: provide cell_id and the complete new_xml in the same compact form (a shape with x, y, w, h; an edge with source and target)
 - For delete: only cell_id is needed
 - Named styles are not available in edit_diagram: write each cell's complete style
 - Find the cell_id from "Current diagram XML" in system context
-- Example update: {"operations": [{"operation": "update", "cell_id": "3", "new_xml": "<mxCell id=\\"3\\" value=\\"New Label\\" style=\\"rounded=1;\\" vertex=\\"1\\" parent=\\"1\\">\\n  <mxGeometry x=\\"100\\" y=\\"100\\" width=\\"120\\" height=\\"60\\" as=\\"geometry\\"/>\\n</mxCell>"}]}
+- Example update: {"operations": [{"operation": "update", "cell_id": "3", "new_xml": "<mxCell id=\\"3\\" value=\\"New Label\\" style=\\"rounded=1;\\" x=\\"100\\" y=\\"100\\" w=\\"120\\" h=\\"60\\"/>"}]}
 - Example delete: {"operations": [{"operation": "delete", "cell_id": "5"}]}
-- Example add: {"operations": [{"operation": "add", "cell_id": "new1", "new_xml": "<mxCell id=\\"new1\\" value=\\"New Box\\" style=\\"rounded=1;\\" vertex=\\"1\\" parent=\\"1\\">\\n  <mxGeometry x=\\"400\\" y=\\"200\\" width=\\"120\\" height=\\"60\\" as=\\"geometry\\"/>\\n</mxCell>"}]}
+- Example add: {"operations": [{"operation": "add", "cell_id": "new1", "new_xml": "<mxCell id=\\"new1\\" value=\\"New Box\\" style=\\"rounded=1;\\" x=\\"400\\" y=\\"200\\" w=\\"120\\" h=\\"60\\"/>"}]}
 
 ⚠️ JSON ESCAPING: Every " inside new_xml MUST be escaped as \\". Example: id=\\"5\\" value=\\"Label\\"
 
 ## Draw.io XML Structure Reference
 
-**IMPORTANT:** You only generate the named styles and the mxCell elements. The wrapper structure and root cells (id="0", id="1") are added automatically. A named style is written before the cells as <mxStyle name="n" value="...style pairs..."/>; a cell uses it by putting the name among its style tokens (see Styles).
+**IMPORTANT:** You only generate the named styles and the mxCell elements. The wrapper structure and root cells (id="0", id="1") are added automatically. A named style is written before the cells as <mxStyle name="n" value="...style pairs..."/>; a cell uses it by putting the name among its style tokens (see Styles). A shape is one self-closing mxCell with x, y, w and h; an edge is one with source and target. vertex="1", edge="1", parent="1" and the mxGeometry element are added automatically, so write them only when needed: parent for a shape inside a container, an mxGeometry element for edge waypoints or a label placed on an edge.
 
 Example - generate ONLY this:
 \`\`\`xml
-<mxCell id="2" value="Label" style="rounded=1;" vertex="1" parent="1">
-  <mxGeometry x="100" y="100" width="120" height="60" as="geometry"/>
-</mxCell>
+<mxCell id="2" value="Label" style="rounded=1;" x="100" y="100" w="120" h="60"/>
 \`\`\`
 
 CRITICAL RULES:
@@ -127,20 +125,16 @@ CRITICAL RULES:
 2. Do NOT include root cells (id="0" or id="1") - they are added automatically
 3. ALL mxCell elements must be siblings - NEVER nest mxCell inside another mxCell
 4. Use unique sequential IDs starting from "2"
-5. Set parent="1" for top-level shapes, or parent="<container-id>" for grouped elements
+5. Write parent="<container-id>" only for shapes inside a container; top-level cells need no parent
 
 Shape (vertex) example:
 \`\`\`xml
-<mxCell id="2" value="Label" style="rounded=1;" vertex="1" parent="1">
-  <mxGeometry x="100" y="100" width="120" height="60" as="geometry"/>
-</mxCell>
+<mxCell id="2" value="Label" style="rounded=1;" x="100" y="100" w="120" h="60"/>
 \`\`\`
 
 Connector (edge) example:
 \`\`\`xml
-<mxCell id="3" style="edgeStyle=orthogonalEdgeStyle;" edge="1" parent="1" source="2" target="4">
-  <mxGeometry relative="1" as="geometry"/>
-</mxCell>
+<mxCell id="3" style="edgeStyle=orthogonalEdgeStyle;" source="2" target="4"/>
 
 ### Edge Routing Rules:
 When creating edges/connectors, you MUST follow these rules to avoid overlapping lines:
@@ -244,7 +238,7 @@ const EXTENDED_ADDITIONS = `
 1. Generate ONLY mxStyle definitions and mxCell elements - wrapper tags and root cells are added automatically
 2. All mxCell elements must be siblings - never nested inside other mxCell elements
 3. Every mxCell needs a unique id attribute (start from "2")
-4. Every mxCell needs a valid parent attribute (use "1" for top-level, or container-id for grouped)
+4. parent defaults to "1"; write it only for a shape inside a container (the container's id)
 5. Edge source/target attributes must reference existing cell IDs
 6. Escape special characters in values: &lt; for <, &gt; for >, &amp; for &, &quot; for "
 
@@ -263,7 +257,7 @@ ${SWIMLANE_EXAMPLE}
 3. Complete the remaining mxCell elements
 4. If still truncated, call append_diagram again with the next fragment
 
-**Example:** If previous output ended with \`<mxCell id="x" style="rounded=1\`, continue with \`;" vertex="1">...\` and complete the remaining elements.
+**Example:** If previous output ended with \`<mxCell id="x" style="rounded=1\`, continue with \`;" x="40" y="40" w="120" h="60"/>\` and complete the remaining elements.
 
 ### edit_diagram Details
 
@@ -289,12 +283,12 @@ edit_diagram uses ID-based operations to modify cells directly by their id attri
 
 Change label:
 \`\`\`json
-{"operations": [{"operation": "update", "cell_id": "3", "new_xml": "<mxCell id=\\"3\\" value=\\"New Label\\" style=\\"rounded=1;\\" vertex=\\"1\\" parent=\\"1\\">\\n  <mxGeometry x=\\"100\\" y=\\"100\\" width=\\"120\\" height=\\"60\\" as=\\"geometry\\"/>\\n</mxCell>"}]}
+{"operations": [{"operation": "update", "cell_id": "3", "new_xml": "<mxCell id=\\"3\\" value=\\"New Label\\" style=\\"rounded=1;\\" x=\\"100\\" y=\\"100\\" w=\\"120\\" h=\\"60\\"/>"}]}
 \`\`\`
 
 Add new shape:
 \`\`\`json
-{"operations": [{"operation": "add", "cell_id": "new1", "new_xml": "<mxCell id=\\"new1\\" value=\\"New Box\\" style=\\"rounded=1;fillColor=#dae8fc;\\" vertex=\\"1\\" parent=\\"1\\">\\n  <mxGeometry x=\\"400\\" y=\\"200\\" width=\\"120\\" height=\\"60\\" as=\\"geometry\\"/>\\n</mxCell>"}]}
+{"operations": [{"operation": "add", "cell_id": "new1", "new_xml": "<mxCell id=\\"new1\\" value=\\"New Box\\" style=\\"rounded=1;fillColor=#dae8fc;\\" x=\\"400\\" y=\\"200\\" w=\\"120\\" h=\\"60\\"/>"}]}
 \`\`\`
 
 Delete container (children & edges auto-deleted):