… STRING results

…ompiler

…s are present

When JPERL_EVAL_USE_INTERPRETER=1, eval STRING was returning undef instead
of the last expression's value in two cases:

1. eval '1; END { }' returned undef because OperatorNode("undef") placeholder
   for the END block overwrote lastResultReg after the real last expression.

2. INIT { eval 'END { }; 1' or die } returned undef because the eval body
   was compiled in VOID context, discarding the result register entirely.

Fixes:
- CompileOperator: OperatorNode("undef") with no operand in VOID context
  is a no-op, not LOAD_UNDEF (END/BEGIN/INIT/CHECK/UNITCHECK placeholders)
- BytecodeCompiler.visit(BlockNode): pre-scan to find last non-placeholder
  statement; freeze lastResultReg after visiting it so trailing placeholders
  cannot clobber it
- BytecodeCompiler.compile(): eval STRING body uses at least SCALAR context
  so the result register is always allocated (never VOID)

… scope snapshot

When JPERL_EVAL_USE_INTERPRETER=1, bare names inside eval STRING (e.g.
*named{CODE}) were resolving to main:: instead of the correct package
at the eval call site (e.g. FOO3::named inside package FOO3 { }).

Root cause: BytecodeCompiler.symbolTable defaulted to 'main' and was
not being updated with the compile-time package/pragma state.

Fix mirrors the JVM compiler's approach exactly:
- CompileOperator: at EVAL_STRING emit time, snapshot the scope manager
  (BytecodeCompiler.symbolTable.snapShot()) and store it as an
  EmitterContext in RuntimeCode.evalContext under a unique evalTag,
  exactly as EmitEval/evalTag does for JVM-compiled eval STRING.
  The evalTag is baked into the EVAL_STRING opcode (4th field).
- SlowOpcodeHandler: read evalTag from opcode, pass to EvalStringHandler.
- EvalStringHandler: retrieve saved EmitterContext from evalContext,
  use its symbolTable snapshot directly as the compile-time scope.
- BytecodeCompiler.compile(): sync package+pragmas from ctx.symbolTable
  so BytecodeCompiler resolves bare names in the eval body correctly.
- RuntimeCode.evalStringWithInterpreter: use capturedSymbolTable
  (compile-time scope from EmitEval) instead of RuntimeCode.getCurrentPackage()
  which uses caller() and cannot see scoped package blocks.

Results: op/stash.t, comp/package_block.t, comp/parser.t now all show
identical pass counts between JVM and interpreter modes.