Canvas combo stepper arrows jump to the first option on numeric combos

Found by @christian-byrne while reviewing #14460, split out because it is pre-existing and out of that PR's scope. Verified independently at HEAD.

## What happens

On the LiteGraph canvas, the increment/decrement arrows on a combo widget whose options are numbers (`[6, 8, 10]`) always land on the **first** option instead of stepping.

## Mechanism

`src/lib/litegraph/src/widgets/ComboWidget.ts:113-121`:

```ts
const foundIndex =
typeof values === 'object'
? indexedValues.indexOf(String(this.value)) + delta
: indexedValues.indexOf(this.value) + delta

const index = clamp(foundIndex, 0, indexedValues.length - 1)
```

`typeof [6, 8, 10] === 'object'` is true for arrays, so the stringified branch runs. But `indexedValues` comes from the local `toArray` (`:25`), which is `Array.isArray(values) ? values : Object.keys(values)` — the numeric array passes through **unstringified**. So `indexOf(String(10))` searches for `'10'` in `[6, 8, 10]` and returns `-1` for every numeric option.

`-1` then produces index `0` in both directions: increment gives `-1 + 1 = 0`, decrement gives `-1 - 1 = -2` which `clamp` raises to `0`.

## Why it is reachable rather than dead

`canUseButton` (`:84`) compares raw values **without** `String()`, so the arrows correctly enable for a mid-list numeric value — and then `tryChangeValue` resets to the first option. The two paths disagree about stringification, which is what makes the broken step reachable in the first place.

It reads as a silent no-op only when the value is already the first option.

## Pre-existing

`git blame` at the merge base `2c937310a3` attributes these lines to `ee222c794b7` (2025-04-28). Not introduced or worsened by #14460, which touches only `WidgetSelect.vue`, `WidgetSelectDefault.vue`, `WidgetSelectDropdown.vue` and tests.

## Related

Same root cause family as #14460 — comparing a stringified option list against an un-stringified value. That PR fixed it in the Vue node path; this is the canvas path.

Read src/lib/litegraph/src/widgets/ComboWidget.ts, especially toArray, canUseButton, and lines 113-121 in tryChangeValue. Reproduce the issue with a numeric combo such as [6, 8, 10], then verify that the increment and decrement arrows move through the numeric options rather than resetting to the first option.

Written by the indexing model from the issue text.