Add HMAC-SHA512 midstate path, vanity PBKDF2 and GPU regression tests

[arkadianet]

Motivation

• Reduce repeated SHA-512 compression work in PBKDF2 by caching HMAC inner/outer midstates for common fixed-length messages.
• Special-case the BIP39 vanity path (salt "mnemonic", 2048 iterations) to get a predictable, faster PBKDF2 path.
• Lower per-work-item private memory by avoiding duplicate padded-key buffers and large temporaries on the hot loop.
• Add GPU and Rust-level regression tests to ensure midstate and vanity paths match the legacy implementations.

Description

• Introduced HmacSha512MidstateCtx and hmac_sha512_midstate_init() plus fixed-length finalizers hmac_sha512_msg12() and hmac_sha512_msg64() in kernels/hmac_sha512.cl to compute HMAC using cached midstates.
• Added pbkdf2_sha512_vanity() in kernels/pbkdf2.cl and switched bip39_entropy_to_seed() in kernels/bip39.cl to call the vanity path, including a safety clamp on password_len.
• Added GPU test kernels: hmac_sha512_midstate_test, hmac_sha512_midstate_msg12_test, and pbkdf2_vanity_test, and corresponding Rust unit tests test_hmac_sha512_midstate_matches_legacy, test_hmac_sha512_midstate_msg12_matches_legacy, and test_pbkdf2_vanity_matches_bip39 in src/kernel.rs to compare against legacy kernels.
• Added small caller-side key hashing for long keys and private-buffer copying for tests to ensure deterministic inputs.

Testing

• Ran cargo test --release as an automated check, but the build failed due to erg-vanity-crypto using the unstable Rust feature bigint_helper_methods on the current toolchain, preventing the test suite from completing.
• The new GPU kernel tests and Rust unit tests were added and will run once the workspace is buildable with the appropriate toolchain or feature flags.
• No GPU regression assertions were observed to run to completion in CI in this rollout because the build error blocked test execution.
• Manual/bench validation of the expected performance and memory improvements is still expected after resolving the build toolchain issue and running the full test suite.

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Codex Task

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