Harden Turbo traversal, arena bounds, and iterable parity - #6538

Summary

• continue recursive native AST walks on fresh stack segments when the C stack runs low
• validate arena offsets and allocation sizes with overflow-safe arithmetic, and reserve records without wrapping the shared cursor
• restore lazy Generator and inner Traversable behavior for combinations, stream native internal combinations, and return a Generator from native template constraints
• add focused parity coverage without committing pathological source fixtures

Verification

• strict C++ build with warnings as errors
• make phpstan
• make tests (22,210 tests; 97,888 assertions)
• Turbo smoke and type-family differential
• 351-class shadow/method parity
• 14,058-method signature parity
• parser differential: 8,770 files, 0 failures
• traversal differential: 615,112 identical events
• cross-process arena smoke
• full Turbo UBSan differential suite
• private stress verification for the three recursive walkers that previously exhausted the native stack