Restrict YClient visibility to prevent queries without key generation

[p0mvn]

Summary

• Bug: YClient methods were pub, allowing external callers to generate queries without first calling generate_secret_keys(). Not regenerating keys for each query could expose an attack that is closed by this PR.
• Fix: Tighten visibility so the only public query API is YPIRClient, which handles key generation internally via generate_secret_keys_from_seed(). All YClient methods are now either fn (private) or pub(crate) (the minimum two that server.rs needs due to Rust lifetime constraints on PolyMatrixRaw<'a>).
• Refactor: scheme.rs benchmarks now use YPIRClient's seed-based API (generate_query_normal / generate_query_simplepir + decode_response_normal / decode_response_simplepir_raw) instead of directly constructing YClient.
• Moved: test_linear_accumulation_noise relocated from noise_analysis.rs to client.rs test module where it has private access to YClient.

Changes

src/client.rs

• YClient::new — pub → pub(crate) (server.rs needs it)
• YClient::generate_query_impl — pub → pub(crate) (server.rs needs it)
• YClient::from_seed, lwe_client, client, generate_query, generate_query_lwe_low_mem, generate_full_query, generate_full_query_simplepir, decode_response — pub → fn (private)
• decode_response_normal_yclient, decode_response_simplepir_yclient — pub → fn (private, wrapped by public YPIRClient methods)
• Added YPIRClient::decode_response_simplepir_raw() — returns Vec<u64> for internal benchmarks
• Added test_linear_accumulation_noise to test module (moved from noise_analysis)

src/scheme.rs

• run_simple_ypir_on_params: uses YPIRClient::generate_query_simplepir + decode_response_simplepir_raw instead of YClient directly
• run_ypir_on_params: uses YPIRClient::generate_query_normal + decode_response_normal instead of YClient directly
• Stores Seed in query tuples instead of YClient references
• Removed unused spiral_rs::client::* import

src/noise_analysis.rs

• Removed test_linear_accumulation_noise (moved to client.rs)
• Cleaned up unused imports (YClient, decrypt_ct_reg_measured, constants::*, barrett_reduction_u128)