keccak_optimized.zig: pure-Zig fallback produces wrong digests

[koko1123]

Discovered once unit tests actually execute (test-harness fix). All 13 keccak_optimized tests fail: the pure-Zig fallback Keccak-256 produces incorrect output (e.g. keccak256("") != c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470). The default XKCP C path is correct and covered; this fallback was broken and undetected because its tests never ran. Debug the permutation against the vectors already in the tests. Tests quarantined with error.SkipZigTest pointing here.