SmallVector: Core SBO storage and basic operations

[jsrivaya]

Parent

Closes part of #5.

Summary

Implement the core small buffer optimization (SBO) storage and basic operations for loon::SmallVector<T, N>.

Scope

This is the foundational issue. All other SmallVector issues depend on this.

Inline storage layout

template <typename T, size_t N>
class SmallVector {
    alignas(T) std::byte inline_storage[sizeof(T) * N];
    T*       heap_ptr  = nullptr;
    uint32_t size_     = 0;
    uint32_t capacity_ = N;

    T* data() { return is_small() ? reinterpret_cast<T*>(inline_storage) : heap_ptr; }
    bool is_small() const { return heap_ptr == nullptr; }
};

Using alignas(T) ensures the inline buffer is correctly aligned for any T, including over-aligned types.

Operations in scope

• push_back(const T&) — copy into next slot; trigger heap transition if at capacity
• pop_back() — destroy last element, decrement size
• operator[](size_t) — bounds-unchecked access
• at(size_t) — bounds-checked access
• data() — raw pointer to storage
• size(), capacity(), empty(), is_small()
• Default constructor, destructor (destroy live elements, free heap if needed)
• clear() — destroy all elements, reset size; keep storage

Heap transition

When size_ == capacity_ and another element is pushed:

1. Allocate new heap block (2x growth)
2. Move all elements from inline storage to heap
3. Destroy originals in inline storage
4. Set heap_ptr, update capacity_

Out of scope for this issue

• Copy/move constructors and assignment (#SmallVector-semantics)
• Iterators (#SmallVector-iterators)
• insert/emplace/erase (#SmallVector-insert)
• Exception safety guarantees (#SmallVector-exceptions)