feat(codegen): register java client plugin

[scarmuega]

Summary

• register java-client as a built-in codegen plugin
• delegate Java generation to the tx3c java-client template
• raise the tx3c compatibility floor to the published 0.25.0 release
• cover parsing, suggestions, CLI help, default output behavior, and the fake-tx3c contract

Verification

• cargo fmt --all -- --check
• cargo build --workspace --locked
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --locked

[scarmuega]

Merge observed.

Plan: plans/java-sdk-trix-registration.md
QA-approved head: 78ca04ccf367a42dfab2f3c5327d2a76e0dac67c

Trellis will verify the human merge evidence, adopt the domain pin, and retire the plan. No further merge action is needed.