fix(core - heap): prevent bypassing Heap invariants

[JalinWang]

Fix heap invariant violations caused by inherited std::vector mutation APIs.

• Use Heap::emplace for group-by and brute-force search insertion.
• Fix DiskANN initial group seeding to target the group heap.
• Make Heap inherit privately and expose explicit read, mutation, and container access APIs.
• Add coverage preventing implicit access to the underlying vector.

[egolearner]

看下heap能改为private继承吗，否则外部还是容易误用

template <typename T, typename TCompare = std::less<T>,
          typename TBase = std::vector<T>>
class Heap : public TBase {

[JalinWang]

看下heap能改为private继承吗，否则外部还是容易误用

template <typename T, typename TCompare = std::less<T>,
          typename TBase = std::vector<T>>
class Heap : public TBase {

done，顺带区分了一下mutable操作的api( mutable_at, mutable_container）

[egolearner]

LGTM