Make JniGen name mangling placement-aware

[milyin]

Summary

• pass the fully qualified destination package to placement-aware JniGen name-mangling hooks; keep the fixed-placement harness hook name-only
• separate package function naming from class method naming with set_method_name_mangle(package, class, name)
• rename class declaration .fun(...) to .method(...) and remove implicit generator-specific class-prefix stripping
• route JNINative externs, handle destructors, vector helpers, class methods, and factories through the contextual method hook
• keep explicit .name(...) overrides verbatim and order-independent

This enables binding consumers such as zenoh-flat-jni to systematize flat Rust names like z_session_put into Session.put without teaching prebindgen a Zenoh-specific convention.

Linked downstream PRs:

• Generate receiver methods from the original zenoh-flat API ZettaScaleLabs/zenoh-flat-jni#2
• Use receiver-style zenoh-flat-jni bindings eclipse-zenoh/zenoh-java#481

Verification

• cargo clippy --all-targets --no-default-features --all-features -- --deny warnings
• cargo test --all --all-features
• bash examples/regen-check.sh
• examples/covertest-kotlin: ./gradlew run (26 sections pass)