feat: capability-lookup wrapper (Phase 1) + public GameTest assertions

[KP2048]

Phase 1 of the pre-release storage/GUI plan: capability-lookup wrapper

Wraps Common Storage Lib's ItemApi/FluidApi/EnergyApi.BLOCK singletons (already
transitively on Archie's classpath, no new Gradle dependency) so ArchieItemStorage/
ArchieFluidStorage/ArchieEnergyStorage can be exposed to third-party mods' pipes and
hoppers on both loaders with one call at registration time — closing the gap called out
explicitly in ArchieEnergyStorage's own KDoc.

New API — net.kernelpanicsoft.archie.transfer.ArchieCapabilityExposure

TileRegistry.TestTile.exposeItemStorage { tile, direction ->
    if (direction == Direction.DOWN) null else tile.items
}

• exposeItemStorage/exposeFluidStorage/exposeEnergyStorage on BlockEntityType<T>,
  each with a direction-aware and a no-direction overload, wrapping CSL's
  BlockLookup.onRegister { registrar -> registrar.registerBlockEntities(...) } (the only
  reachable path for BlockEntityGetter-based registration — confirmed via javap against
  the real CSL jars; registerSelf only accepts Block-keyed BlockGetter).
• RegistrySupplier<BlockEntityType<T>> convenience overloads via Architectury's .listen{},
  for calling right next to a DeferredRegister declaration.
• Stretch: Item.exposeItemStorage via ItemApi.ITEM, for item-in-item interop with other
  CSL-depending mods.
• Deliberately not auto-wired into NBTHolder.itemField/etc. — registration must happen
  once per BlockEntityType, while those delegates run per-instance; an explicit call at
  registration time avoids awkward static bookkeeping.

Verification

New CapabilityLookupTests/CapabilityLookupTestFixtures in Archie-Test, registered at
mod-init time (after TileRegistry.init(), mirroring real consumer-mod usage — CSL's
onRegister is a one-shot listener that fires during CSL's own platform registration event,
long before a GameTest server boots, so this can't live inside the @GameTest methods
themselves). All 3 pass under a real fabric-test:runGametest run:

[GameTest] PASS archie:capabilitylookuptests.testexposeitemstoragereachesitemapiblock
[GameTest] PASS archie:capabilitylookuptests.testexposeitemstoragerespectsdirection
[GameTest] PASS archie:capabilitylookuptests.testunexposedtypeisnotfound

Manual-only (not GameTest-coverable): a real hopper/pipe mod on both Fabric and NeoForge
pulling/pushing into a block exposing ArchieItemStorage, before release.

Also in this PR: make GameTestAssertions.kt public

assertTrue/assertEquals/expectThrows were internal to Archie's own common module,
so Archie-Test's new tests above couldn't use them despite being the obvious sugar for this
kind of test. Moved from gametest.internal.tests to the public net.kernelpanicsoft.archie.gametest
package (alongside AClientGameTestHarness.kt) and made public; updated the 3 server-side
test files that referenced them via same-package resolution to import explicitly, and updated
docs/gametest.md/AGENTS.md to match.

Part of the three-feature pre-release plan (capability lookup / data attachments / item-backed
container menus); Phases 2 and 3 to follow in separate PRs.

🤖 Generated with Claude Code

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