[C API] Implement custom allocator interface and related functions for memory management

[rfsaliev]

No description provided.

[Copilot]

🟡 Changes recommended

The public allocator interface breaks compatibility, while callback validation, error propagation, and lifetime documentation remain unsafe.

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Pull request overview

Adds configurable built-in and custom allocators to C API Vamana index construction and loading.

Changes:

• Introduces the C custom allocator interface and builder configuration APIs.
• Propagates allocator handles through static/dynamic graph and data storage.
• Adds allocator accounting tests and improves dynamic memory estimates.

File summaries

File	Description
include/svs/orchestrators/dynamic_vamana.h	Adds allocator-aware dynamic builds.
include/svs/index/vamana/dynamic_index.h	Builds dynamic graphs with supplied allocators.
include/svs/core/graph.h	Makes graph loading allocator-aware.
include/svs/core/allocator.h	Generalizes erased allocator rebinding.
bindings/c/tests/c_api_test_utils.h	Adds a tracking test allocator.
bindings/c/tests/c_api_index.cpp	Tests static-index allocator configuration.
bindings/c/tests/c_api_dynamic_index.cpp	Tests dynamic-index allocator configuration.
bindings/c/src/svs_c.cpp	Implements C allocator setters.
bindings/c/src/index_builder.hpp	Stores and forwards allocator handles.
bindings/c/src/dispatcher_vamana.hpp	Extends static dispatcher declarations.
bindings/c/src/dispatcher_vamana.cpp	Applies allocators to static data and graphs.
bindings/c/src/dispatcher_dynamic_vamana.hpp	Extends dynamic dispatcher declarations.
bindings/c/src/dispatcher_dynamic_vamana.cpp	Applies allocators and updates estimates.
bindings/c/src/data_builder/sq.hpp	Uses allocator handles for SQ data.
bindings/c/src/data_builder/simple.hpp	Uses allocator handles for simple data.
bindings/c/src/data_builder/lvq.hpp	Uses allocator handles for LVQ data.
bindings/c/src/data_builder/leanvec.hpp	Uses allocator handles for LeanVec data.
bindings/c/src/allocator.hpp	Adapts C callbacks to C++ allocators.
bindings/c/include/svs/c/svs_c.h	Exposes the allocator C API.

Review details

• Files reviewed: 19/19 changed files
• Comments generated: 6
• Review effort level: Balanced

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