Apply drag position corrections inside React Flow instead of after it

[dovvnloading]

Problem

The canvas adjusts every drag position before rendering it: the drag-speed factor scales the movement, smart guides snap it to alignment, and dragging a group carries its members by the same delta. Those adjustments ran in onNodesChange, which is downstream of React Flow's own bookkeeping, so the library never learned about them. The file documented the consequence in its own drag-end comment: the corrections "never feed back into RF's drag state."

That left two different positions for one node on every frame of every drag - the corrected position this app rendered the card at, and the raw pointer position the library still believed. Connection geometry is computed by the library, from its own node records. So the card and the line attached to it were being positioned from two different numbers. That is the connection failing to track the node, and it explains why the symptom survived every downstream fix attempted so far: re-render tuning, layer promotion, culling, and compositing changes all operate after the point where the two values have already diverged, and none of them can make two different numbers agree.

Change

The corrections now run as a React Flow change middleware (experimental_useOnNodesChangeMiddleware), inside the library's own position update, before anything is committed:

• The corrected position becomes the only position. The card's placement and the connection's endpoints are derived from one number, in one commit.
• Group-member movements join the same batch, so a group and its members stay in lockstep inside the library's update rather than alongside it.
• React Flow's drag-stop change receives the identical correction. This also removes the release-time bounce that the previous code had to paper over by substituting the settled position after the fact.
• onNodesChange no longer computes positions at all. It applies the corrected batch to local state and runs the side effects a settled gesture owes the rest of the app: the backend position commit, the guide lines, and the culling suspension.

Test plan

• Full frontend check passes: schema, types, lint (0 errors), 1927/1927 tests, production build, bundle size
• The drag-size-cache test now drives production's real sequence - middleware first, then the handler with its output - rather than the handler alone
• On the affected machine: drag a connected node slowly and confirm the connection stays attached to it