Retry signed transaction broadcast across known nodes

[ElectronicUtility]

Summary

• broadcast the same already-signed transaction through the preferred node followed by the saved known-node list
• normalize and de-duplicate node URLs while preserving preferred-first order
• stop on the first non-empty transaction ID and preserve the last concrete node error when every attempt fails
• add focused JVM coverage for ordering, fallback, early stop, and all-nodes-failed behavior

Fixes #186.

Verification

• ./gradlew :common-jvm:test — 24 tests passed, 0 failures
• git diff --check

The repository currently pins ergo-appkit_2.11:44fddd97-SNAPSHOT, which is no longer available from its configured remote repositories. For the local test run I built and published that exact upstream commit to mavenLocal; no dependency change is included in this PR.

[ElectronicUtility]

Implementation is ready for review. The official reservation is valid at ErgoDevs/Ergo-Bounties#69, and the full common-jvm test suite passed locally (24 tests, 0 failures) against the repository's exact pinned AppKit commit. I am available to address any requested changes promptly.