Fix uninitialized class/struct members in public headers

[maxwbuckley]

Summary

• Initialize POD members with sensible defaults to prevent undefined behavior when users create instances without explicitly setting all fields
• Fix base_filter to have a virtual destructor for safe polymorphic deletion
• Clean up empty constructor syntax ({}; → {}) and use = default for trivial destructors

Files Changed

File	Changes
cluster/agglomerative.hpp	min_samples = 5
distance/distance.hpp	KernelParams: kernel = LINEAR, degree = 3, gamma = 1.0, coef0 = 0.0
distance/grammian.hpp	cublas_handle = nullptr, constructor/destructor cleanup
neighbors/ball_cover.hpp	index_trained = false
neighbors/common.hpp	virtual ~base_filter(), num_ranks_ = 0
neighbors/hnsw.hpp	ef = 200
neighbors/vamana.hpp	pq_codebook_size = 0, pq_dim = 0
preprocessing/spectral_embedding.hpp	n_components = 2, n_neighbors = 15, norm_laplacian = true, drop_first = true

Test plan

• Verify existing tests pass (no behavioral change expected for code that explicitly sets these fields)
• Confirm default values are reasonable based on existing documentation/usage patterns

🤖 Generated with Claude Code

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