feat: add Java SDK generated-client journey

[scarmuega]

Adds journey 12-java-sdk: generate bindings with trix's built-in Java plugin and tx3c template, compile the generated Maven artifact and a clean Java 21 host against land.tx3:tx3-sdk:0.15.0, then resolve, sign, submit, and confirm a transfer on a disposable local devnet funded with the public CIP-1852 test vector.

Dedicated stable/beta jobs run on ubuntu-24.04 with Temurin 21 and Maven 3.9. They enforce tx3c >= 0.25.0 and trix >= 0.28.0 so a version-gated skip cannot count as coverage. Cleanup targets only the journey's own Dolos configuration; failure artifacts remain under the existing harness contract.

Verification at bf69e16af108224c28a7b9d48a8bba7305a26d03:

• Released stable journey passed generation, generated/consumer Maven verification, Central consumption of SDK 0.15.0, resolve/sign/submit/confirmation. Fresh Maven repository; no snapshot override.
• Stable Java CI and beta Java CI executed the journey with tx3c 0.25.0 and trix 0.28.0: one pass, zero skips each.
• Full local suite completed successfully, including Java and TypeScript 07-codegen-consume. Hydra internally skipped without Docker; live-network internally skipped without secrets. Those skips are not runtime coverage.
• Cleanup probe terminated the owned process and preserved an unrelated process; no Dolos daemon remained after the suite.
• Shell syntax, Maven XML, workflow YAML, and diff checks passed.

Local isolated-channel caveat: unset an inherited TX3_ROOT before running the harness, since tx3up prioritizes it over the temporary HOME. The initial attempt failed channel discovery after updating the ambient stable installation; the clean retry passed. No common harness change is included.

The previous Maven Central publication blocker is resolved. This PR remains draft for the declared QA stage.

[scarmuega]

Merge observed.

Plan: plans/java-sdk-dx-e2e.md
QA-approved head: bf69e16af108224c28a7b9d48a8bba7305a26d03

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