[compiler] Fix JSX tags with underscore-prefix incorrectly treated as host components

[amitsaroj]

Summary

Fixes #36601

In lowerJsxElementName (BuildHIR.ts), the check used to distinguish user-defined component tags from host (builtin) element tags was:

if (tag.match(/^[A-Z]/)) {
  // treat as component — load from scope
} else {
  // treat as BuiltinTag (host element like <div>, <span>)
}

The JSX convention (and the React runtime) is: a tag is a host element if and only if its first character is a lowercase ASCII letter [a-z]. Any other first character — including _, $, or an uppercase letter — means the tag is a user-defined component.

The old regex ^[A-Z] only matched tags beginning with an uppercase letter, so tags like <_Bar> or <$Special> fell through to the BuiltinTag branch and their values were never tracked as dependencies. This produced silently incorrect memoization — _Bar was treated as a stable builtin tag even though it is a prop that could change between renders.

Fix

Change the condition to its logical inverse:

// before
if (tag.match(/^[A-Z]/)) {

// after  
if (!tag.match(/^[a-z]/)) {

This preserves the existing behaviour for the common cases:

• div, span, button, … → still BuiltinTag (start with [a-z])
• • Button, MyComponent, … → still component (start with [A-Z], not [a-z])
    And now correctly handles:
• _Bar, _Foo → component (not [a-z] → now takes the component path) ✓
• • $Special → component (not [a-z]) ✓

Test plan

Added a regression test fixture at:
compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/jsx-underscore-prefix-component.js

Run the compiler test suite to generate the updated snapshot:

yarn test --filter ReactCompiler
# or, to regenerate snapshots:
yarn test --filter ReactCompiler --update-snapshots

Repro (from issue #36601)

function Foo({ _Bar }) {
  return <_Bar>{() => <div />}</_Bar>;
}

Before fix: _Bar was treated as a BuiltinTag. The JSX was memoized with zero dependencies — changes to _Bar across renders were silently ignored.

After fix: _Bar is correctly treated as a component reference, loaded from scope and tracked as a dependency.

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