SmallVector: Iterator support

[jsrivaya]

Parent

Part of #5. Depends on #71.

Summary

Implement full iterator support for SmallVector compatible with std::vector iterator requirements.

Scope

SmallVector stores elements contiguously (inline buffer or heap — both flat arrays), so raw pointers are valid random-access iterators.

using iterator               = T*;
using const_iterator         = const T*;
using reverse_iterator       = std::reverse_iterator<iterator>;
using const_reverse_iterator = std::reverse_iterator<const_iterator>;

iterator begin()  { return data(); }
iterator end()    { return data() + size_; }
// const, cbegin, cend, rbegin, rend, crbegin, crend variants

Invalidation rules (must be documented and tested)

Operation	Invalidates iterators?
push_back — no reallocation	No
push_back — triggers heap transition	Yes, all
pop_back	Only end()
insert / emplace	From insertion point to end()
erase	From erasure point to end()
clear	All
Move assignment	All (into destination)

STL algorithm compatibility

With raw pointer iterators, std::sort, std::find, std::copy, std::ranges::* all work out of the box. Add value_type, size_type, difference_type, pointer, reference type aliases for full Container concept compliance.

Testing requirements

• Range-based for loop
• std::sort on SmallVector
• std::copy into SmallVector via std::back_inserter
• Iterator remains valid after push_back within inline capacity
• Iterator invalidated after heap transition