Fix if/unless scope warning + analyze local package variable bug

[fglock]

Summary

Fix: Spurious 'my variable masks earlier declaration' warning

Variables declared in if (my $x = ...) conditions now correctly create a new scope, matching Perl's behavior:

• my $x = 1; if (my $x = 2) {...} - no warning (different scopes) ✓
• if (my $x = 1) {...} elsif (my $x = 2) {...} - warns correctly (same if-chain scope) ✓

Before: TAP::Formatter::Session showed spurious warnings during jcpan test runs.
After: No spurious warnings.

Analysis: Root cause of Log4perl/Carp test failures

Discovered that local $Foo::X from outside package Foo doesn't affect $X accessed inside Foo subroutines:

package Foo;
our $X = 0;
sub check { print "X=$X\n"; }

package main;
local $Foo::X = 1;
Foo::check();  # jperl: "X=0", Perl: "X=1"

Root Cause: our variables cache a reference to the scalar object in a register at declaration time. When local replaces the global with a new object, the subroutine's cached register still points to the old object.

Impact: Breaks $Carp::CarpLevel and affects 16+ Log4perl tests.

Design Doc

Added dev/design/local-package-variable-fix.md with detailed fix plan:

• Recommends re-fetching our variables from global symbol table on each access
• 5-phase implementation plan
• Risk assessment and timeline (1-2 days)

Changes

• src/main/java/org/perlonjava/frontend/parser/StatementParser.java - Add scope for if/unless conditions
• dev/design/log4perl-compatibility.md - Document root cause analysis
• dev/design/local-package-variable-fix.md - New fix plan document

Test Plan

• my $x = 1; if (my $x = 2) {...} no longer warns
• if (my $x = 1) {...} elsif (my $x = 2) {...} still warns correctly
• Full test suite passes (make)
• No spurious warnings from TAP::Formatter::Session

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