v2.7→v2.8 parity: automated Linux and Windows release builds

[alexeid]

Part of v2.7→v2.8 parity (raised by Remco). Umbrella: #34.

Problem

v2.7 shipped stand-alone Linux (tar.gz) and Windows (.exe installer) releases. v2.8 currently only has an automated Mac build.

Current state

• release/Mac/build-sign-dmg.sh — complete jpackage pipeline with code signing and DMG output (~650 lines).
• release/Windows/ — launch4j XML configs (11 files) and .bat wrapper scripts present, but no build script.
• release/Linux/jrebin/ — shell wrappers present, but no build script.
• pom.xml release profile (lines 311–345) only configures GPG signing for Maven Central, not OS installers.

Work

• Write release/Windows/build-exe.sh analogous to the Mac script (jpackage or launch4j-based, with bundled JRE).
• Write release/Linux/build-tar.sh to produce a tar.gz with bundled JRE and launcher scripts.
• Wire into a top-level release procedure (Makefile or docs section in README).

Acceptance

• One command (per OS) produces a signed, distributable installer/archive.
• README documents the release procedure for all three platforms.