(Python) f-string nested replacement field in a format spec closes `subst` at the wrong brace

**Which language:** Python (specified explicitly, not auto-detect)
**Version:** 11.12.0
**Using:** `highlight` (not `highlightAuto`)

**Describe the issue**
A nested replacement field inside an f-string format spec (dynamic width/precision, e.g. `f"{x:{w}}"`) closes the `subst` region on the *inner* `}`. The field's own closing `}` is then emitted as plain string text, and the `subst` span ends up brace-unbalanced.

**Sample Code to Reproduce**
```python
f"{x:{w}}"
```

**Actual** — `hljs.highlight('f"{x:{w}}"', {language:'python'}).value`:
```html
f"{x:{w}}"
```
The `subst` span is `{x:{w}` (an unmatched `{`) and the final `}` renders in string color instead of as part of the field. `f"{value:{width}.{prec}f}"` fails the same way.

**Expected behavior**
The whole `{x:{w}}` is a single replacement field, and the nested `{w}` is itself a field. This is valid Python — `{w}` sets the width dynamically (`f"{x:{w}}"` with `x=42, w=6` → `' 42'`). The closing `}` should belong to the field, not leak into the string, and the `subst` region should be brace-balanced.

**Cause**
`SUBST` in `src/languages/python.js` (`begin:/\{/, end:/\}/`) does not `contain` itself (`SUBST.contains = [STRING, NUMBER, PROMPT]`), so it ends at the first inner `}` and cannot handle nested replacement fields.

Assisted-by: Claude Opus 4 (high)

Start in src/languages/python.js, inspect the SUBST grammar and reproduce the issue with the f-string examples using highlight with the Python language selected. The work is done when nested replacement fields remain brace-balanced, the whole {x:{w}} field is highlighted correctly, and the final } no longer renders as plain string text.

Written by the indexing model from the issue text.