Optimize hash_pbkdf2() by reusing HMAC contexts

[gregPDN]

Fixes #9604.

This optimizes hash_pbkdf2() in ext/hash by avoiding repeated HMAC key-block hashing work across PBKDF2 rounds.

What changed

• Reuse precomputed HMAC base contexts in PBKDF2 loops (ipad/opad pre-hash once, copy per round).
• Add a generic copy-based HMAC helper for PBKDF2 (php_hash_hmac_round_with_copy).
• Add SHA-256 PBKDF2 fast path in ext/hash.
• Add SHA-256 helper PHP_SHA256Final32FromContext() for fixed-size inner/outer follow-up rounds.
• Add regression/vector test: ext/hash/tests/gh9604.phpt.

Functional behavior

No public API changes. hash_pbkdf2() input validation and output semantics are preserved.

Validation

• make test TESTS="ext/hash/tests/hash_pbkdf2_basic.phpt ext/hash/tests/hash_pbkdf2_error.phpt ext/hash/tests/gh9604.phpt"
• make test TESTS="ext/hash/tests"

Benchmark

Benchmark script (CLI) used:

• hash_pbkdf2('sha256', 'password', 'salt', 200000, 64, false)
• 9 timed runs per sample (hrtime(true)), median reported.

Environment:

• macOS 15.x arm64
• php-src master built with --disable-all --enable-cli

Result summary on this machine:

• Before optimization: median 135.47 ms
• After optimization: median of 12 samples 28.28 ms (sample medians ranged 27.68–28.46 ms)
• Speedup: ~4.79x

[gregharmon]

Superseded by #21175 (same patchset from the correct account). Please review #21175.

[gregharmon]

Superseded by #21175 (opened from my main account). Please ignore this one and review #21175.