chart: Avoid quadratic Sankey topology computation (#2682)

# Title

chart: Avoid quadratic Sankey topology computation

# Body

Closes #2681

## Description

`Sankey::topology` previously computed node ranks in repeated waves.
Each wave allocated an `N`-element boolean vector and scanned all node
indices to build the next frontier. A valid chain of `N` nodes therefore
required `O(N^2)` work for both the forward and backward rank passes,
blocking the UI while the chart was painted.

This change replaces that loop with a degree-based topological
traversal. Each node and link is processed once per direction, reducing
rank computation to `O(V + E)` while preserving the existing
longest-path `depth` and `height` values and circular-link detection.

The public API and chart rendering flow are unchanged.

## Screenshot

| Before | After |
| ------ | ----- |
| <img width="1972" height="1262" alt="sankey-pre-fix-reproduction"
src="https://github.com/user-attachments/assets/3b97270e-b056-429e-831e-8beadc935842"
/> | <img width="1972" height="1262" alt="sankey-post-fix-verification"
src="https://github.com/user-attachments/assets/0c88a56a-3c47-45bf-b248-c921ce0d8a6e"
/> |

With the same optimized PoC against the checked-in `sankey.rs`:

- Before: a valid 50,000-node chain exceeded the 5-second timeout
(`exit_code=124`).
- After: the same chain completed in 5 ms (`exit_code=0`); a same-size
shallow star completed in 4 ms.

## How to Test

```sh
cargo test --locked --offline --all
cargo clippy -p gpui-component --lib -- --deny warnings
cargo run -p gpui-component-story -- Chart
```

The automated test suite includes a 50,000-node chain regression case
for `Sankey::topology`.

For the manual story check, open the Chart story and scroll to the
`Sankey Chart - TSLA` examples.

## AI Assistance

AI assisted with the implementation, regression test, and PR draft. The
contributor reviewed the generated changes and test evidence before
submission.

## Checklist

- [x] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [X] Reviewed the changes in this PR and confirmed AI generated code
(if any) is accurate.
- [x] Passed `cargo run` for story tests related to the changes.
- [x] Not platform-specific; no platform-specific rendering path was
changed.

---------

Co-authored-by: Floyd Wang <gassnake999@gmail.com>