Expose opaque-tier fields needed by generated JNI

[milyin]

Summary

• preserve the original base / expanded two-tier source structure
• add only the missing opaque z_* accessors needed for one-crossing JNI callback decomposition
• add numeric-id encoding construction and with_schema on the opaque encoding tier
• expose borrowed key-expression text and Rust error messages to language adapters
• fix the existing unstable-feature throughput example call

Ownership

No existing session or key-expression signature changes. In particular, z_open(ZConfig) still consumes its configuration exactly like zenoh::open, and declaration functions still consume their ZKeyExpr inputs.

This is the flat source for ZettaScaleLabs/zenoh-flat-jni#2 and eclipse-zenoh/zenoh-java#481.

Validation

• cargo check --all-features
• cargo test --all-targets --all-features
• clippy with the two existing main-branch lints allowed: module inception from the original layout and manual is-multiple-of in the existing throughput example
• full downstream 101-test JVM suite against prebindgen main

[milyin]

Coordinated follow-ups: ZettaScaleLabs/zenoh-flat-jni#2 and eclipse-zenoh/zenoh-java#481.