fix: Remove wax unwrap usage (#12899)
## Why
`wax` still required a crate-level `.unwrap()` allowance, hiding
implementation unwraps behind the workspace panic-hardening exemption.
Closes TURBO-5625
## What
Replaces implementation unwraps with slice matching or explicit
invariant `expect()`s, then narrows the crate-level allowance to
`expect_used` only.
## How
- `cargo fmt -p wax`
- `cargo test -p wax`
- `cargo clippy -p wax --all-targets -- -D warnings`
- Pre-push hook passed with format, check:toml, and Rust checks
