Preserve filehandle line counter assigned through localized $. #1965

Summary

arXiv::FileGuess 1.01 fails its focused t/arxiv/fileguess.t test on PerlOnJava because dynamically localizing $.' restores the previous filehandle line counter when the scope exits. System Perl preserves the counter value assigned to $.' in this case.

The test rewinds a file after reading its \\documentclass line, assigns local $. = 0 in a short block, then scans for \\pdfoutput=1 within the next five lines. On PerlOnJava, the filehandle counter returns to 10 after the block; rereading the fixture counts that marker as line 24, so detection incorrectly returns TYPE_LATEX2e instead of TYPE_PDFLATEX.

Evidence

• Distribution: arXiv 1.01 (arXiv::FileGuess), pure Perl.
• System Perl 5.42.2: the complete upstream suite passes; focused test passes.
• PerlOnJava 5.44.1: upstream run fails 1 of 44 assertions; focused test reproduces on both JVM and interpreter backends.
• Archived CPAN run: 20261104-113007-26634.
• A focused line-counter reproduction confirms PerlOnJava restores the saved RuntimeIO.currentLineNumber from ScalarSpecialVariable.dynamicRestoreState() after the localized scope; system Perl leaves the assignment effective for the handle.

Please add project-owned regression coverage and make localized $.' match Perl's per-handle behavior.