Replace DefaultHBaseCluster with DefaultStorageBackendFactory

[em3s]

Summary

Replace DefaultHBaseCluster with DefaultStorageBackendFactory + HBaseStorageBackend (both already on main via PRs #191, #194, #192).

Before: DefaultHBaseCluster -- a monolithic singleton bundling connection management, mock branching, and table access.

After: DefaultStorageBackendFactory dispatches to pluggable backends (HBaseStorageBackend, MemoryStorageBackend, MockHBaseStorageBackend), each implementing the StorageBackend interface. The HBase backend can now be swapped for any other StorageBackend implementation.

Storage backend series: PR #191 -> PR #194 -> PR #192 (all merged) -> this PR

Known issue

This PR was started before the v3 abstractions landed on main, so it created v2-local duplicates (StorageBucket, DatastoreUri, DefaultStorageBackendFactory, etc.) instead of using the v3 classes directly. These should be removed and replaced with v3 imports.

Remove (v2 duplicate)	Use instead (v3, on main)
v2/.../StorageBackend	engine.storage.StorageBackend
v2/.../StorageBucket / StorageBuckets	engine.storage.StorageTable
v2/.../DatastoreUri	engine.storage.DatastoreUri
v2/.../DefaultStorageBackendFactory	engine.storage.DefaultStorageBackendFactory
v2/.../HBaseStorageBackend	engine.storage.hbase.HBaseStorageBackend
v2/.../MockHBaseStorageBackend	engine.storage.hbase.MockHBaseStorageBackend
v2/.../MemoryStorageBackend	engine.storage.memory.MemoryStorageBackend

Test plan

• ./gradlew :engine:build passes
• CI passes

[em3s]

Code Review Round 3 - RESOLVED

• (High) DefaultStorageBackendFactory.close() not synchronized, instance0 not nulled → added @Synchronized, null out instance0 on close
• (High) MemoryStorageBackend.getTable() throws eagerly instead of Mono.error() → changed to Mono.error()
• (High) HBaseStorageBackend.connectionMono is public → made private, added getAdminMono() public method
• (Medium) HBaseOptions.getBuckets()/getTables() log at INFO on every call → changed to DEBUG

Tracked separately:

• Rename HBaseRecord to StorageRecord #187 — rename HBaseRecord → StorageRecord

All issues resolved in commit 3c04c7c

Reviewed by claude code (opus 4.6)

[em3s]

Too large to review; splitting it up now.

[em3s]

PR Split Plan

This PR (26 files, +1517/-143) has been split into 3 incremental PRs for easier review:

Part 1: Core abstractions + Memory backend — #191

Branch: feat/storage-backend-interfaces → main

• StorageBackend interface: getStorageTable(namespace, name) → Mono<StorageTable> (URI overload as default method)
• StorageTable interface, DatastoreUri (strict validation: ^[a-z0-9_]+$)
• MemoryStorageTable, MemoryStorageBackend implementation
• StorageTableCompatibilityTest abstract contract test suite
• All tests for the above
• Purely additive, no existing code modified

Part 2: HBase backend + Factory — #192

Branch: feat/storage-backend-hbase → feat/storage-backend-interfaces (rebases to main after Part 1 merges)

• HBaseStorageTable, HBaseStorageBackend, MockHBaseStorageBackend — all implement getStorageTable()
• DefaultStorageBackendFactory
• HBaseTestingStorageBackend test fixture
• HBaseTestingClusterExtension modification (additive init call)
• All tests for the above
• Mostly additive, one test fixture modification

Part 3: Consumer migration — This PR (#175)

After Parts 1+2 merge to main, this PR rebases and shrinks to only the consumer migration:

• Graph.kt — swap DefaultHBaseCluster → DefaultStorageBackendFactory
• GraphDefaults.kt — remove datastore field
• DatastoreHashLabel.kt, DatastoreIndexedLabel.kt — use factory's getStorageTable()
• HBaseOptions.kt — rewrite with getStorageTable()
• HBaseDatastoreBindingConfiguration.kt — use factory
• HBaseTestingClusterExtension.kt — remove old init
• Delete compat/DefaultHBaseCluster.kt
• Pure swap + cleanup

Merge order

PR #191 → main → PR #192 → main → PR #175 (now small) → main

[em3s]

Plan: Remove v2 duplicates, wire to v3 StorageBackend

PR #175 was started before v3 storage abstractions landed on main (PRs #191/#194/#192), so it created v2-local duplicates. This cleanup removes them and wires v2 engine directly to v3 engine.storage.*.

Key design: HBaseTablesProvider interface

v2 Labels need Mono<HBaseTables> for multi-column HBase operations (Filters, CellUtil), but v3 StorageBackend only returns Mono<StorageTable>. A new HBaseTablesProvider interface bridges this gap:

interface HBaseTablesProvider {
    fun getHBaseTables(namespace: String, name: String): Mono<HBaseTables>
}

Implemented by: HBaseStorageBackend, MockHBaseStorageBackend, HBaseTestingStorageBackend

Changes summary

Action	Files	Description
NEW	1	HBaseTablesProvider interface
MOD (v3)	5	Add HBaseTablesProvider impl + getAdminMono() to backends, widen DatastoreUri regex
MOD (v2)	6	Switch Graph, HBaseOptions, DatastoreHashLabel, DatastoreIndexedLabel, HBaseTestingClusterExtension, DefaultHBaseClusterTest to v3 imports
MOD (server)	1	Switch HBaseDatastoreBindingConfiguration to v3 imports
DELETE	17	v2-local duplicate src (10) + test (7) files

Files to delete

StorageBackend.kt, StorageBucket.kt, StorageBuckets.kt, DatastoreUri.kt, DefaultStorageBackendFactory.kt, HBaseStorageBackend.kt, HBaseStorageBucket.kt, MockHBaseStorageBackend.kt, MemoryStorageBackend.kt, MemoryStorageBucket.kt + 7 test files