Engine: centralize FExecuting re-entrancy guard into a TGocciaIteratorHelperValue base class

[frostney]

Summary

PR #579 added an FExecuting: Boolean re-entrancy guard to all 8 iterator helper classes (concat, map, filter, take, drop, flatMap, zip, zipKeyed) per ES2026 §27.1.2.1.1. The guard is currently duplicated across 8 field declarations and 16 AdvanceNext/DirectNext methods — identical check + set + try-finally in every one.

Problem

Adding a new iterator helper requires remembering to copy the pattern. Missing it produces a SIGSEGV on re-entrant .next() (the bug #514 demonstrated).

Proposed fix

Introduce TGocciaIteratorHelperValue as an intermediate class between TGocciaIteratorValue and all 8 helper classes:

TGocciaIteratorValue
  └── TGocciaIteratorHelperValue  (new: owns FExecuting, BeginExecuting/EndExecuting)
        ├── TGocciaConcatIteratorValue
        ├── TGocciaLazyMapIteratorValue
        ├── TGocciaLazyFilterIteratorValue
        ├── TGocciaLazyTakeIteratorValue
        ├── TGocciaLazyDropIteratorValue
        ├── TGocciaLazyFlatMapIteratorValue
        ├── TGocciaZipIteratorValue
        └── TGocciaZipKeyedIteratorValue

The base class would own FExecuting and provide a protected BeginExecuting (check + set + throw) and EndExecuting (clear). Each subclass still needs the try-finally to call EndExecuting, but the field, the check, the throw, and the error message reference live in one place.

Non-helper iterators (array, string, map-entry, set-entry, generator, generic) are unaffected — they continue inheriting directly from TGocciaIteratorValue.

Scope

• source/units/Goccia.Values.IteratorValue.pas — declare the new class
• source/units/Goccia.Values.Iterator.Concat.pas
• source/units/Goccia.Values.Iterator.Lazy.pas (5 classes)
• source/units/Goccia.Values.Iterator.Zip.pas (2 classes)

No JS-visible behavior change. Existing re-entrancy tests in tests/built-ins/Iterator/iterator-helper-reentrant.js serve as the verification suite.

Related

• Engine SIGSEGV: Iterator.concat with generator mid-next #514 — the SIGSEGV that motivated the per-class guards
• Fix iterator helper SIGSEGV: re-entrancy guard, generator Close, AcquireExceptionObject (#514) #579 — the PR that introduced the current duplicated pattern