fix(resolver): stop corrupting single-segment use const imports

[AlessioGiacobbe]

Problem

parseUse rebuilds the imported constant name by splitting on the last backslash, but strrpos() returns false for a single-segment name, and substr($id, false + 1) drops the first character:

namespace App;

use const PHP_EOL;    // registered as `\HP_EOL`

echo PHP_EOL;         // compiled binary: Undefined constant "HP_EOL"

The mangled name is visible verbatim in the runtime error of the compiled program.

Fix

$id is already the fully qualified constant name — store it directly and drop the split/re-join. For multi-segment imports the removed recomposition produced the identical value, so nothing else changes.

Tests

• New tests/compiler/namespace/use-const-single-segment.phpt: a namespaced file importing the global PHP_EOL plus a cross-namespace use const App\ANSWER. Expected output verified against PHP 8.4; on master the compiled binary dies with Undefined constant "HP_EOL".
• tests/compiler/namespace/ suite: 34/34 pass. No PHPStan errors on the touched file.