feat: skribe-fuzz-rs fuzzing loop implementation

[gtrepta]

Blocked on:

• kframework_ffi: Fixes and Improvements kframework-rs#11
• Fix Str marshalling kframework-rs#12

Implements the core fuzzing loop end-to-end: parses a fuzz spec, selects the target contract/function, substitutes ABI-encoded calldata into the KORE template, runs the semantics, and checks the exit code.

Changes

• Updated kframework-rs dependency from 1e7ee3a to 73c4986
• Completed the main fuzz loop in fuzz_target_1.rs, wiring together argument parsing, spec loading, marshalling, K framework execution, and exit code checking
• Implemented SignatureAbi::arbitrary_input to generate random, ABI-encoded calldata from a function's parameter types using DynSolValue::arbitrary_from_type