fix(sankey): keep explicitly-positioned and snapped nodes inside the plot area

[CAOShurong]

Summary

Fixes #7946 — Sankey nodes are clipped at the bottom edge when positioned with node.x / node.y, or when a snap collision cascade walks a column past height.

There are two independent causes, both addressed:

1. Explicit positioning centers the node on y * height without bounds checking, so a node requested at y = 1 renders half outside the plot area (plotly.js/src/traces/sankey/render.js, "Force node position"). The fix clamps the node rect to [0, height] when it would overflow either edge.
2. snap collision cascade only ever pushes overlapping nodes downward, so a clustered column gets walked straight off the bottom edge even when there is empty space above to absorb the correction. The fix adds a bottom-bounded upward pass (resolveCollisionsBottomToTop) run after the existing top-to-bottom pass.

Test plan

• Added a jasmine regression test (test/jasmine/tests/sankey_test.js) reproducing both cases from the issue:
  • explicit position with arrangement: "fixed" (node at y = 0.98) — asserts the node rect stays inside the plot area;
  • arrangement: "snap" with an overlapping column — asserts no node rect overflows the bottom edge.
• Local run of the sankey suite: the two new specs pass. (The drag-interaction specs fail under headless Chrome in this environment regardless of this change — pre-existing, unrelated to the fix; full CI on the PR validates the rest.)
• A standalone geometry harness exercising the exact algorithm passes both repros (pre-fix overflow of 61.9px on Repro 1 → 0 after the fix; Repro 2's previously-invisible node now renders at the bottom edge).

Notes

• Behavior for non-edge positions and arrangement: "snap" columns with room above is unchanged.
• No visual-baseline images change (positions only shift for the previously-clipped edge cases).