Fix interpreter $_ variable in foreach loops

[fglock]

Summary

Two bugs fixed in the bytecode interpreter for $_ in foreach loops.

Bug 1: local $_ produced $main::main::_ instead of $main::_

The local operator handler was manually concatenating packageName + "::" + name, but when $_ is parsed its IdentifierNode.name is already main::_, producing the double-qualified name. Fixed by using NameNormalizer.normalizeVariableName() consistently (6 sites).

Bug 2: $_ not aliased to loop element in foreach loops

The interpreter's For1Node handler stored the next iterator element into a register but never aliased it to $main::_, so $_ was always undefined inside the loop body.

Solution: two new superinstructions

• LOCAL_SCALAR_SAVE_LEVEL rd levelReg nameIdx (302): atomically saves getLocalLevel() into levelReg before calling makeLocal(name). The pre-push level is used by POP_LOCAL_LEVEL after the loop to correctly restore $_ at any nesting depth.

• FOREACH_GLOBAL_NEXT_OR_EXIT varReg iterReg nameIdx exitTarget (304): per-iteration superinstruction combining hasNext + next() + aliasGlobalVariable(name, element) + conditional exit.

• POP_LOCAL_LEVEL rs (303): restores DynamicVariableManager to the saved pre-push level after loop exit.

Safe coordination

visit(BlockNode) detects the BlockNode([local $_, For1Node(needsArrayOfAlias)]) pattern and skips the local $_ child using a local variable skipFirstChild (not a mutable field), so For1Node owns the full local/loop/restore sequence.

Verified

for (1..3) { say $_ }                              # 1 2 3
$_ = 123; for (1..3) { show() } show()             # 1 2 3 123
# nested foreach loops restore $_ correctly