dillon1000/react

[compiler] Fix JSX tags prefixed with `_` or `$` incorrectly treated as host elements (#36688)

## Summary

Fixes #36601

### Problem

In `lowerJsxElementName` (BuildHIR.ts), the condition `if
(tag.match(/^[A-Z]/))` only treats JSX tags starting with an **uppercase
letter** as component references. Tags starting with `_`, `$`, or any
other non-letter character fell through to the `else` branch and were
incorrectly classified as `BuiltinTag` (host/intrinsic elements).

This meant that `<_Bar />` or `<$Foo />` was treated like `<div />`,
causing the compiler to skip memoization of the component and
potentially producing incorrect output.

### Root Cause

JSX semantics (as implemented by Babel's JSX transform) are:
- Tag starts with **lowercase** letter → host/intrinsic element (string
tag)
- **Everything else** → component reference (in-scope identifier)

The original code only handled the first half of that rule ("starts with
uppercase → component") while ignoring identifiers like `_Bar` and
`$Foo`.

### Fix

Change:
```ts
if (tag.match(/^[A-Z]/)) {
```
To:
```ts
if (!tag.match(/^[a-z]/)) {
```

This correctly classifies any JSX identifier that does NOT start with a
lowercase letter as a component reference, matching JSX spec semantics.

### Test

Added fixture `jsx-underscore-prefix-component` that renders `<_Bar />`
and verifies the compiler correctly memoizes it as a component
reference.

## How did you test this change?

- Added new compiler fixture test: `jsx-underscore-prefix-component`
- Ran `yarn workspace babel-plugin-react-compiler lint` ✅
- Ran snap tests with the new fixture to generate expected output ✅