refactor(bootstrap)!: make minestom extensions a platform module

[TheMeinerLP]

Stacked PR

This PR is stacked on #403 (fix(runtime): close the bean scope when the startup check fails), branch fix/close-scope-on-failed-start. The diff against that branch is only this change. Merge #403 first, or retarget this PR to main after it lands.

What

Minestom is no longer a hard dependency of the lobby bootstrap. A variant now gets exactly one ServerBootstrap, chosen at start:

• core: new ServerBootstrap contract (name, init(Auth), start(host, port), loadedExtensions()).
• runtime: ServerBootstraps.select picks the one candidate, plain Minestom (PlainMinestomBootstrap) when there is none, and aborts naming all candidates when there are several. TitanStartup fixes the order: init the server, install the exception handler, build the Avaje scope, start the server.
• platform/extensions (new): ExtensionServerBootstrap on minestom-extensions, registered through META-INF/services.
• platform/cloudnet (new): the CloudNet deliver (MessageChannelDeliver), chosen by the .wrapper directory and marked @Primary.
• runtime keeps DebugDeliver as a @Secondary fallback. TracedDeliver moves to common so both sides wrap the deliver the same way.
• apps/cloudnet includes extensions, cloudnet and luckperms. apps/local includes none of them.
• platform/luckperms reads the loaded extensions from the injected ServerBootstrap and no longer depends on minestom-extensions.
• README: the start log line Server bootstrap: <name> and the per-variant behaviour.
• Behaviour change: a bind failure in bootstrap.start(...) now logs and exits with status 1. Previously it propagated out of main, and the JVM could hang on non-daemon threads.

Why

runtime depended on the extension loader, so every variant loaded extensions/, including the local development server. The server init order was only implicit in main.

BREAKING CHANGE: only the apps/cloudnet variant loads the extensions folder; apps/local starts plain Minestom without the extension loader.

Deviations from design.md

• ServiceLoader instead of a second Avaje scope (D2): as designed.
• Module check through LoadedModules: each ServerBootstrap found on the loader counts as <name>Platform (so extensionsPlatform). This is used by the boot check and by the test-side verification alike.
• CloudNetEnvironment stays in common: runtime (profile selection) and ConnectorStartupCheck need it, so moving it would invert the dependency. It gained an isPresent(Path) overload for the tests.
• TracedDeliver moves to common: not in the design. platform/cloudnet has to wrap its deliver in the same trace, and must not depend on runtime.
• platform/cloudnet sets its own group: it shares the name cloudnet with apps/cloudnet, and Gradle resolved the platform dependency to the app project (circular task graph). Found while wiring the variant.
• ServerBootstrap.loadedExtensions() returns an ordered List instead of a Set, so the start log and checks stay stable.
• Titan's constructor takes the bootstrap: new Titan(bootstrap); the bootstrap goes into the scope as a bean.

How it was tested

Unit (F.I.R.S.T., no server, no real working directory, no system properties):

• ServerBootstrapsTest: no candidate falls back to plain Minestom; one candidate is chosen; two abort naming both.
• TitanStartupTest: init, exception handler, scope, start in that order; a failing scope never starts the server; the start log line Server bootstrap: {} is captured through a ListAppender.
• BootstrapSettingsTest (characterisation, task 1.4): forwarding secret file before property, blank file ignored, missing file falls back, bind host and port defaults and overrides, non-numeric port falls back to 25565.
• ExtensionNamesTest, ExtensionServerBootstrapRegistrationTest: stable name order; the ServiceLoader registration.
• CloudNetDeliverSelectionTest (@TempDir): .wrapper selects MessageChannelDeliver, without it DebugDeliver.
• MessageChannelDeliverTest (moved, now uses a ListAppender), TracedDeliverTest (moved).
• DeliverFallbackBeansTest, LuckPermsPermissionServiceStartTest (a LuckPerms extension reported by the bootstrap aborts the start).
• ExpectedModulesTest: extensionsPlatform is reported missing under plain Minestom.
• Variant starts: apps/cloudnet VariantStartTest asserts the active bootstrap is extensions and extensionsPlatform is loaded; apps/local asserts minestom.

Verification:

• ./gradlew spotlessApply build --no-daemon passes.
• ./gradlew :apps:cloudnet:bootSmokeTest :apps:local:bootSmokeTest --rerun --no-daemon passes.
• openspec validate --all: see the archive commit.

Scenario mapping (task 4.5, read-only check)

Every server-bootstrap scenario maps to at least one test (the extension-loader and socket scenarios are partial). Not covered by unit tests, all of them need a live process: jars in extensions/ loaded before the first player connects, jars ignored in apps/local, forwarding enforced in both variants, the bind address applied to the socket. These are the manual checks in 4.4. The read-only check also noted that TitanStartupTest attaches its ListAppender to a shared logger, the same pattern VariantStartupCheckTest already uses; it detaches in finally.

Unticked tasks (manual, need a live server)

• 2.3: start with ExtensionServerBootstrap and an empty extensions/ in a @TempDir. MinecraftServer.init is process-global and the loader reads extensions/ relative to the working directory, so it cannot run as a unit test. Covered by 4.4 instead.
• 4.4: manual E2E acceptance. titan-cloudnet.jar with the CloudNet bridge and the Titan bridge in extensions/: both load, the log shows Server bootstrap: extensions, the navigator transfer works inside CloudNet, and the chat message appears outside it. titan-local.jar with a filled extensions/: nothing loads, the log shows Server bootstrap: minestom. forwarding.secret and -Dservice.bind.port work in both.

Task 5.1 (this PR) is ticked after the PR is opened, as in #403.

https://claude.ai/code/session_01GXose9rakuRNZoM3TP3xj1