Note

Medium Risk
Touches low-level storage and key semantics (including panics/None behavior and compaction/shrink logic), so subtle correctness regressions could impact arena-backed structures, but changes are well-covered by expanded tests for stale-key invalidation.

Overview
Augments the generational_slab fork with generational indexing by introducing a (position, generation) Key and storing generations per slot, so lookups/removals with stale keys fail instead of aliasing reused slots (ABA prevention).

This is a breaking API shift: most slab APIs and iterators now use Key instead of usize, get_disjoint_mut gains a GenerationMismatch error, and internal behaviors update generation bookkeeping (including a generation_watermark to invalidate old keys across clear/drain/compact/shrink_to_fit). The PR also removes the serde feature and FromIterator<(usize, T)> path (deleting the builder/serde modules), updates numeric_range_tree’s NodeArena/NodeIndex to wrap the new Key, and refreshes tests/snapshots (including memory usage changes).

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