refactor: remove bump field from ChunkIter

[overlookmotel]

Minor simplification.

It's unnecessary for ChunkIter to contain a PhantomData<&'a mut Bump> because it also contains a ChunkRawIter<'a, MIN_ALIGN> - and that contains PhantomData<&'a mut Bump>. This is sufficient to "hold" the lifetime for both iterators.

[Copilot]

Pull request overview

This PR aims to simplify ChunkIter by removing its redundant bump: PhantomData<&'a mut Bump> field, relying on ChunkRawIter to carry the lifetime instead.

Changes:

• Remove the bump: PhantomData<&'a mut Bump> field from ChunkIter
• Simplify iter_allocated_chunks to construct ChunkIter with only its raw iterator

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[Copilot]

Removing bump: PhantomData<&'a mut Bump<MIN_ALIGN>> from ChunkIter weakens the borrow the iterator holds to an immutable borrow via ChunkRawIter (ChunkRawIter currently uses PhantomData<&'a Bump<MIN_ALIGN>>). Since Bump has allocation APIs that take &self (interior mutability), callers can allocate while a safe ChunkIter is alive, which can invalidate the returned chunk slices and lead to UB. Keep the &'a mut Bump phantom in ChunkIter, or otherwise ensure the iterator type enforces exclusive access for the duration of iteration.

Suggested change

	raw: ChunkRawIter<'a, MIN_ALIGN>,
	raw: ChunkRawIter<'a, MIN_ALIGN>,
	bump: PhantomData<&'a mut Bump<MIN_ALIGN>>,

[Copilot]

The safety comment here (“Ensured by mutable borrow of self.”) becomes incorrect if ChunkIter no longer carries a PhantomData<&'a mut Bump<MIN_ALIGN>>: the iterator only keeps an immutable borrow, and Bump exposes mutation via &self (e.g., alloc, try_alloc_layout, etc.). This means allocations can occur while the safe iterator is alive, violating the safety preconditions for producing &'a [MaybeUninit<u8>].

[overlookmotel]

Copilot's critique sounds reasonable! I'm not actually sure he's right, but keeping the bump field is more explicit. Closing.