fix(free-layout-core): recompute line path when version unchanged but path is empty

[zyh94946]

Summary

In the free layout editor, some connection lines fail to render (their SVG path
stays empty) after loading a graph via fromJSON. It only happens on initial load —
dragging nodes or creating connections manually works fine. Same problem as #1070.

Root cause

WorkflowLineRenderData.update() skips recomputation when the position version is
unchanged (a performance optimization), and path comes from the line contribution:
get path() { return this.currentLine?.path ?? ''; }.

When a line entity is created before its line contribution (bezier/fold/straight,
registered by free-lines-plugin in onReady) is available:

1. The first update() writes the real coordinates into version, but currentLine
   is still undefined, so path is left empty.
2. Once the contribution registers, the coordinates are unchanged, so the recomputed
   version equals the stored one.
3. oldVersion === newVersion is true, the guard returns early, currentLine.update()
   is never called, and the empty path is locked in permanently — the line never
   renders.

This matches #1070's observation that it's specific to initial fromJSON loading
(manual drag/connect happens after contributions are registered).

Fix

Skip recomputation only when the version is unchanged and a path has already been
computed:

if (oldVersion === newVersion && this.currentLine?.path) {
  return;
}

A line whose path is still empty is recomputed once its contribution becomes
available. Lines that already have a non-empty path are unaffected, preserving the
original optimization.

Testing

Added __tests__/workflow-line-render-data.test.ts:

• Regression test reproducing the late-contribution case — verified red/green:
  fails without the fix (path stays ''), passes with the fix.
• Guard test: a redundant update() on a line that already has a path is a no-op.

rush build --to @flowgram.ai/free-layout-core succeeds; free-layout-core tests all
pass (86 cases, incl. the 2 new ones).

Refs #1070

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