slab: iterator caches the bitmap word #3

slab::iterator copies the current 64-bit word (slab.cppm:288) and consumes bits from the copy. Later changes to that word are invisible:

• Neighbour erased after the iterator was built: still set, ++it yields a destroyed slot.
• Neighbour inserted after: not set, skipped. pool::erase then returns the wrong next.

multislab and pool iterators wrap it, so both inherit it.

Repro (confirmed)

libmem::pool<int, 64> p;
p.insert(1);
auto b = p.insert(2);
auto it = p.begin();
p.erase(b);
++it;           // != end(), points at destroyed b

libmem::pool<int, 64> p;
auto a = p.insert(1);
p.insert(2);
p.erase(a);     // returns end(), 2 is live

Fix

Store the bit index instead of the word. On increment, re-read slab_->bitmap_[word_idx_] and mask off bits <= bit_. Costs one L1 load per step. operator== compares indices; make_iterator(index) no longer pre-masks.

Tests

Both repros, with an element type that marks itself dead in its destructor.