[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 ✅

compiler/packages/babel-plugin-react-compiler/src/HIR/BuildHIR.ts

@@ -3409,7 +3409,7 @@ function lowerJsxElementName(
   const exprLoc = exprNode.loc ?? GeneratedSource;
   if (exprPath.isJSXIdentifier()) {
     const tag: string = exprPath.node.name;
-    if (tag.match(/^[A-Z]/)) {
+    if (!tag.match(/^[a-z]/)) {
       const kind = getLoadKind(builder, exprPath);
       return lowerValueToTemporary(builder, {
         kind: kind,

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/jsx-underscore-prefix-component.expect.md

@@ -0,0 +1,56 @@
+
+## Input
+
+```javascript
+// Test that JSX tags starting with `_` or `$` are treated as component
+// references (not host/builtin elements). JSX semantics: any tag NOT starting
+// with a lowercase letter is a component reference.
+
+import {Stringify} from 'shared-runtime';
+
+const _Bar = Stringify;
+
+function Component(props) {
+  return <_Bar value={props.value} />;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{value: 42}],
+};
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime"; // Test that JSX tags starting with `_` or `$` are treated as component
+// references (not host/builtin elements). JSX semantics: any tag NOT starting
+// with a lowercase letter is a component reference.
+
+import { Stringify } from "shared-runtime";
+
+const _Bar = Stringify;
+
+function Component(props) {
+  const $ = _c(2);
+  let t0;
+  if ($[0] !== props.value) {
+    t0 = <_Bar value={props.value} />;
+    $[0] = props.value;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  return t0;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ value: 42 }],
+};
+
+```
+      
+### Eval output
+(kind: ok) <div>{"value":42}</div>

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/jsx-underscore-prefix-component.js

@@ -0,0 +1,16 @@
+// Test that JSX tags starting with `_` or `$` are treated as component
+// references (not host/builtin elements). JSX semantics: any tag NOT starting
+// with a lowercase letter is a component reference.
+
+import {Stringify} from 'shared-runtime';
+
+const _Bar = Stringify;
+
+function Component(props) {
+  return <_Bar value={props.value} />;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{value: 42}],
+};