Prepare wasip2 certificate envelope verification

[jasisz]

Summary

• add verifier-side parsing for the reserved wasip2ComponentEnvelope declaration
• add a wasip2 artifact-prep helper that hashes the delivered component but slices the embedded core only by manifest-declared lengths
• validate the delivered component and declared core slice for well-formedness without using component parsing to locate the core
• keep schema 5 behavior unchanged: target = "wasip2" still rejects before envelope parsing or WebAssembly validation

Refs #1146

Tests

• cargo fmt --all -- --check
• cargo clippy -p aver-cert --all-targets -- -D warnings
• cargo test -p aver-cert
• cargo test -p aver-cert wasip2 -- --nocapture
• git diff --check