SPL: LimitIterator::seek(null) soft-null E_DEPRECATED (#31676)

[PurHur]

Summary

• LimitIterator::seek(null) now follows php-src Z_PARAM_LONG: E_DEPRECATED for parameter #1 ($offset), then soft-coerce to 0 (no TypeError outside strict_types).
• Shared helper: VmMath::parseZParamLongBuiltinArgForFrame (same path as #31621 construct soft-null).
• PHP implementation: ext/spl/LimitIteratorBuiltin.php — no new runtime/*.c.
• php-src ref: ext/spl/spl_iterators.c zim_LimitIterator_seek.

Closes #31676

Test plan

• Issue repro (VM):

  ./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_limititerator_seek_null.php'
  

  Output: E_DEPRECATED … #1 ($offset) + cur=10 key=0
• ./script/phpunit.sh --filter Issue31676LimitIteratorSeekNullTest — OK (4 tests: VM/JIT soft + strict TypeError)
• Compliance case: test/compliance/cases/stdlib/limititerator_seek_null_soft.phpt
• Not run: full ci-local / compliance suite / bootstrap link (narrow SPL arg coerce)

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