Net::SMTP support: fix $+ alternation bug and IO::File::new_tmpfile recursion

[fglock]

Summary

Net::SMTP (libnet 3.15) CPAN tests now pass: 19/19 test programs, 110/110 subtests.

Two bugs were fixed:

• Fix $+ (LAST_PAREN_MATCH) for regex alternations: lastCaptureString() was returning the highest-numbered capture group regardless of whether it participated in the match. Now iterates backwards to find the first non-null group, matching Perl 5 semantics. This fixes Net::Netrc's token parser which uses $+ to extract quoted/unquoted tokens.

• Fix IO::File::new_tmpfile infinite recursion: The pure-Perl new_tmpfile() called $class->new which causes StackOverflowError when subclasses override new() to call new_tmpfile(). Replaced with bless gensym(), $class to match the XS behavior (no polymorphic dispatch through subclass new).

Files Changed

• src/main/java/org/perlonjava/runtime/regex/RuntimeRegex.java — lastCaptureString() method
• src/main/perl/lib/IO/File.pm — new_tmpfile() method
• dev/modules/net_smtp.md — Support plan document

Test plan

• ./jcpan -t Net::SMTP — All 19 test programs pass (110/110 subtests)
• make — All unit tests pass (no regressions)
• $+ returns correct group in alternations: "test" =~ /(a)|(test)|(c)/ gives $+ = "test"
• IO::File::new_tmpfile works with subclasses that override new()

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