floating bars incorrectly offset by stacked base value

### Expected behavior

Floating bars should be rendered at their absolute y coordinates regardless of the stacked configuration. Since a floating bar already encodes both its start and end position explicitly, it should not be offset by the cumulative stack value of other datasets.

### Current behavior

When using floating bars (`y: [start, end]`) with `stacked: true` on a linear axis, some datasets are not rendered at all when multiple datasets share the same x value. This happens because the stacked base offset calculated by `applyStack` is incorrectly added to the absolute coordinates of the floating bar, pushing affected bars outside the visible scale range.

### Reproducible sample

https://jsfiddle.net/g9kLhmbe/2/

### Optional extra steps/info to reproduce

_No response_

### Possible solution

Add a `floatingBarMode` option at the dataset level to distinguish between absolute and relative positioning of floating bars.

`'absolute'`: the floating bar is rendered at its exact `[start, end]` coordinates, ignoring any stacked offset.
`'relative'`: the floating bar is offset by the cumulative stack value of preceding datasets, preserving the current behavior.

To avoid a breaking change, `floatingBarMode` could default to `'relative'` to preserve existing behavior.

### Context

_No response_

### chart.js version

v4.5.1

### Browser name and version

_No response_

### Link to your project

_No response_