Implement Perl defer blocks (use feature 'defer')

[fglock]

Summary

Implements Perl's defer statement from use feature 'defer', which schedules code blocks to run at scope exit in LIFO order.

Key Changes

• DeferBlock class: Implements DynamicState to integrate with DynamicVariableManager cleanup
• DeferNode AST node: New AST node for defer statements
• Parser support: parseDeferStatement() in StatementParser, registered via StatementResolver
• JVM backend: EmitStatement.emitDefer() compiles closure and pushes DeferBlock to DVM
• Bytecode interpreter: PUSH_DEFER opcode (377) for deferred execution
• Scope detection: FindDeclarationVisitor.containsLocalOrDefer() triggers cleanup for blocks with defer

Bug Fixes

goto &sub fixes

• Fixed goto &sub parser: Parser was incorrectly transforming goto &sub into return &sub(@_), causing return inner() to be wrongly detected as a tail call. Now properly produces goto operator.
• Fixed goto &{expr} tail calls: goto &{"subname"} (symbolic code dereference) was calling the function and using the return value as goto target instead of performing a tail call. Added handleGotoSubroutineBlock() in EmitControlFlow.java and corresponding support in interpreter.
• Fixed goto &sub in $SIG{DIE}: Added TAILCALL trampoline in WarnDie.java to handle tail calls in die handlers.
• Improved error messages: "Goto undefined subroutine" error message now matches Perl format for goto &sub with undefined subroutines.

Interpreter parity fixes

• Fixed interpreter crash with tied() in scalar context: When tied() was used inside a ternary operator, the interpreter crashed with ClassCastException.
• Fixed vec lvalue in interpreter: Added vec case in CompileAssignment.java for interpreter support.
• Fixed |= and ^= operators: Changed to use polymorphic bitwiseOr/bitwiseXor instead of numeric-only ops.

Test Results

Test	Status
Unit tests	PASS (163 tests)
op/die_goto.t	PASS (5/5)
uni/goto.t	PASS (2/4) - remaining 2 fail due to pre-existing regex bug

Regression Analysis

All reported regressions have been investigated:

• Fixed: op/die_goto.t, uni/goto.t tests 1-2
• Pre-existing on master (not regressions): op/bop.t, op/warn.t, re/subst.t, re/pat_rt_report.t (MethodHandle conversion errors)

Known Limitations

• last, next, redo don't trigger defer cleanup when jumping out of scope (documented in test)

Test plan

• Unit tests pass (./gradlew test)
• defer.t test file with 14 test cases
• op/die_goto.t passes (5/5)
• uni/goto.t tests 1-2 pass (goto &{expr})
• Manual testing with both JVM and interpreter backends

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