wasm-mutate: add global mutator

[GtechGovind]

Summary

• add an AddGlobalMutator that appends a zero-initialized numeric global
• preserve existing global sections and insert new sections in canonical order
• keep the mutation semantics-preserving and disable it in reduction mode
• cover empty modules, existing globals, section ordering, and applicability

Part of #415.

Testing

• cargo test -p wasm-mutate
• cargo clippy -p wasm-mutate --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check