feat(installer): install extensions by default — REPL cross-index JOINs & MVs out of the box

[fupelaqu]

Problem

A fresh REPL install could not run cross-index JOINs at all:

1. The softclient4es{N}-cli assembly contains no JOIN engine (zero DuckDB/Arrow classes — verified by jar inspection). JOINs live in softclient4es-arrow-extensions, discovered via the ExtensionSpi ServiceLoader.
2. The launcher ran java -jar <assembly>, which ignores the classpath entirely — manually dropping jars into lib/ did nothing.
3. The extensions are thin jars needing their full dependency closure (~256 jars — Apache Arrow, DuckDB native, Flight, …). "Just add a couple of jars" was never going to work.

Changes (install.sh + documentation/client/repl.md)

• Classpath launcher: java -cp "<assembly>:lib/*" app.softnetwork.elastic.client.Cli, assembly first so it wins any conflict; Arrow --add-opens flags added automatically on Java 9+.
• Extensions installed by default, resolved with their full dependency closure via a bootstrapped coursier launcher (single portable jar, runs on the already-required JVM):
  • community extensions (materialized views): always — any engine version (matching 0.1.x line selected for engines < 0.20), Java 8+;
  • arrow extensions (cross-index JOINs): engines ≥ 0.20, Java 11+ — a hard runtime constraint, not a policy choice: Apache Arrow 18.3.0 ships Java-11 bytecode (class-file major 55, verified on the published jars), so on Java 8 the JOIN engine cannot load. On Java 8 the installer says so and still installs MVs.
• --no-extensions for a minimal install; VERSION file + summary report what was installed.
• repl.md gains an Extensions section: default behaviour, manual cs fetch recipe for existing installs, the thin-jar warning, per-extension Java requirements.

Verified end-to-end (live ES 8.18.3, Docker)

Fresh install → CREATE TABLE ×2 → INSERT fixtures → SELECT … FROM jdbc_join_emp e JOIN jdbc_join_dept d ON e.dept_id = d.dept_id → correct INNER JOIN rows (dept names populated, orphan dropped). 277 jars in lib/ (~488 MB).

Depends on / uncovered

• SOFTNETWORK-APP/softclient4es-arrow#125 — E2E testing exposed an extension priority collision: EnforcedDqlExtension (priority 50, greedy canHandle) swallows JOIN queries before JoinExtension (also 50), silently returning wrong single-index rows. Fixed there; needs an arrow-extensions 0.2.1 release, which this installer picks up automatically (latest-version resolution). Until 0.2.1 is published, a default install still exhibits the collision.
• Two more bugs to file separately: silent wrong JOIN results when no join extension is present (core hardening), and REPL JOIN … ORDER BY failing on qualifier loss.
• install.ps1 (Windows) parity is a follow-up.

⚠️ Note before merge: this script is fetched from main at install time by end users — merging changes the public install behaviour immediately. Recommended order: merge arrow#125 → release arrow-extensions 0.2.1 → merge this.

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