=== symfony-0001: PropertyAccessor::writeCollection O(P*C) -> O(P+C) ===

                 scale    defective      fixed    speedup
------------------------------------------------------------
  P=  100 C=  100       0.27ms     0.05ms      5.2x
  P=  300 C=  300       3.72ms     0.22ms     16.8x
  P=  500 C=  500      10.53ms     0.38ms     27.6x
  P= 1000 C= 1000      43.75ms     0.85ms     51.8x
  P= 2000 C= 2000     183.87ms     2.08ms     88.4x

Conclusion: dual-lookup hoist (SplObjectStorage + serialize-keyed array).
Symfony Doctrine entities with deep OneToMany hit O(N^2) without this fix.
