Update cargo-fuzz setup; split fuzz_value_sanitize so it runs

- Fix compiler errors in `clarity/fuzz/` and refresh dependencies.
- Move `fuzz_value_sanitize` into its own fuzz target so `cargo-fuzz` runs it.

Notes:
- Now builds and runs on `+nightly`.
- I have not fully reviewed semantic validity; kept the diff minimal.
- Prelude for #6355 (Expand input coverage with fuzzing).

How to:
- List: `cargo +nightly fuzz list`
- Build: `cargo +nightly fuzz build [target]`
- Run: `cargo +nightly fuzz run fuzz_value_sanitize`

Author: moodmosaic