Migrate to React Flow 12 and fix rendering regressions

.gitignore

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 dist-ssr
 *.local
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 # Claude Code
 .claude

docs/react-flow-12-review.md

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+# React Flow 12 migration review
+
+The migration is implemented on `chore/react-flow-12`, based on `main` at
+`9de921a`. React Flow 11.11.4 is replaced by `@xyflow/react` 12.12.0. React,
+Node, build tooling, and model-file format version 2 remain unchanged. Version 1
+models remain supported. Merging and releasing are separate steps.
+
+After switching branches, run `npm install` and restart the development server
+so the installed dependencies match the branch.
+
+## Implementation and follow-up fixes
+
+- Migrate imports, styles, chunking, internal-node geometry, and viewport APIs.
+- Keep runtime measurements out of serialization, dirty tracking, and undo.
+  Preserve explicit area dimensions and gesture-level resize history.
+- Wait for export node measurements and asynchronous viewport changes; support
+  annotation-only exports without waiting for nonexistent nodes.
+- Preserve the fixed measurements of derived association helpers so associative
+  links remain visible after updates and view changes.
+- Make sidebar focus exclusive across nodes and edges in all views, and add
+  focus-button tooltips with keyboard visibility.
+- Render composition diamonds in the edge SVG without clipped tips. Composite
+  the line and diamond before applying opacity to prevent overlap artifacts.
+
+## Validation on 22 October 2026
+
+- `npm run lint`: passed.
+- `npm run test:unit`: 541 tests passed in 35 files. The original 514 tests remain.
+- `npm run build:ci`: passed. The existing MySQL parser chunk-size warning remains.
+- The initial migration comparison against `main`, recorded in commit `f88f7bc`,
+  covered a 20-class diagram, light/dark rendering, all three views, matching
+  conceptual PNG preview dimensions, pen undo/redo, and an annotation-only preview.
+- Follow-up browser checks confirmed solid composition diamonds with matching
+  line colors in light and dark themes.
+- A temporary browser fixture using the actual model-state hook, node/edge
+  components, and PNG exporter confirmed the reported associative link renders.
+  Real generated PNG bytes were saved locally and visually inspected:
+  conceptual and logical output at 1919 × 1185, physical output at 1919 × 1275,
+  and annotation-only output at 1734 × 1185. These fixtures included negative
+  coordinates, text, shapes, ink, and notes/areas omitted from the live canvas.
+  Content remained unclipped, and the physical default-values footer was visible.
+  Dark-mode export retained light export styling and annotation colors.
+- Personal model files and local review images are not part of the commits.
+
+## Outstanding manual review
+
+Browser download-event automation timed out, so completion of a native PNG
+download through the production dialog remains unverified. The PNG rendering
+itself was verified independently from its generated image bytes.
+
+Exhaustive keyboard, pointer, touch, area-resize, relationship-control-point,
+and annotation-tool checks across all views and themes remain a release review
+task. Automated regressions cover these areas in part; they do not establish
+complete visual or interaction parity. Browser file-picker automation also
+failed for the supplied model; file parsing/opening has automated coverage,
+and the model rendered through the local review fixture.
+
+## Deferred class panel layout
+
+Discussed with the user and explicitly deferred until after the migration:
+
+- Align attribute fields using consistent label and input columns.
+- Clarify N/UQ/AI controls, their selected states, and explanatory tooltips.
+- Reduce uppercase label noise and excessive vertical spacing.
+- Move the class color control into the header instead of a separate Options section.
+- Consider compact, accessible delete actions in class and attribute headers,
+  retaining class deletion confirmation.
+- Add muted type/key summaries to collapsed attributes.
+- Keep the colored class stripe, expandable attributes, and Add attribute placement.
+
+Start with alignment, spacing, and the color control. These are recorded design
+ideas, not changes included in this migration.