build: unify codegen on rkyv 0.8

[pathscale]

Moves worktable_codegen from rkyv 0.7.45 to the same rkyv 0.8 line used by WorkTable and DataBucket.

Verification:

• cargo check -p worktable_codegen
• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• full test run: 521 passed, one torn-shutdown test failed once and passed immediately when rerun exactly; the changed dependency is compile-time codegen support only.