all: Replace tiny-keccak with alloy::primitives::keccak256

tiny-keccak 2.0 dropped the top-level keccak256 function and changed
Keccak's streaming API, breaking every call site. Route all keccak
hashing through alloy::primitives::keccak256, which is already used
elsewhere in the codebase, and drop the tiny-keccak dependency.