Add wasip2 envelope wall vocabulary

[jasisz]

Summary

• add checker-owned Wasip2Envelope.lean vocabulary for a declared-length prefix ++ embedded_core_module ++ suffix split
• register the new wall source/pristine root and bump the wall identity/snapshot
• fix the WASM clippy failure from the wasip2 component-envelope parser by using if let for the single matched payload
• document that the vocabulary is reserved for future wasip2 acceptance and is not imported into schema 5 acceptance

Refs #1146

Tests

• cargo fmt --all -- --check
• cd aver-cert/assets/wall/current && lake env lean Wasip2Envelope.lean
• cargo clippy -p aver-lang --lib --bin aver --features wasm,wasip2 -- -D warnings
• cargo test -p aver-cert
• cargo test --features wasip2 --test wasip2_codegen_regression component_ -- --nocapture
• cargo test --features wasm,certify --test cert_certify_spec certify_add_one_output_is_unchanged_when_the_int_helper_is_present -- --nocapture
• cargo test --features wasm,certify --test cert_verify_spec cert_tripwire_declines_unsupported_artifact_target -- --nocapture