Add Kotlin Multiplatform SDK (JVM + iOS)

sdks/kotlin/DEVELOP.md

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+# Kotlin SDK — Developer Guide
+
+Internal documentation for contributors working on the SpacetimeDB Kotlin SDK.
+
+## Project Structure
+
+```
+src/
+  commonMain/     Shared Kotlin code (all targets)
+    com/clockworklabs/spacetimedb/
+      SpacetimeDBClient.kt    DbConnection, DbConnectionBuilder
+      Identity.kt             Identity, ConnectionId, Address, Timestamp
+      ClientCache.kt          Client-side row cache (TableCache, ByteArrayWrapper)
+      TableHandle.kt          Per-table callback registration
+      SubscriptionHandle.kt   Subscription lifecycle
+      SubscriptionBuilder.kt  Fluent subscription API
+      ReconnectPolicy.kt      Exponential backoff configuration
+      Compression.kt          expect declarations for decompression
+      bsatn/
+        BsatnReader.kt        Binary deserialization
+        BsatnWriter.kt        Binary serialization
+        BsatnRowList.kt       Row list decoding
+      protocol/
+        ServerMessage.kt      Server → Client message decoding
+        ClientMessage.kt      Client → Server message encoding
+        ProtocolTypes.kt      QuerySetId, QueryRows, TableUpdateRows, etc.
+      websocket/
+        WebSocketTransport.kt WebSocket lifecycle, ping/pong, reconnection
+  jvmMain/        JVM-specific (Gzip via java.util.zip, Brotli via org.brotli)
+  iosMain/        iOS-specific (Gzip via platform.zlib)
+  commonTest/     Shared tests
+  jvmTest/        JVM-only tests (compression round-trips)
+```
+
+## Architecture
+
+### Connection Lifecycle
+
+```
+DbConnectionBuilder.build()
+  → DbConnection constructor
+    → WebSocketTransport.connect()
+      → connectSession() opens WebSocket
+        → processSendQueue()   (coroutine: outbound messages)
+        → processIncoming()    (coroutine: inbound frames)
+        → runKeepAlive()       (coroutine: 30s idle ping/pong)
+```
+
+On unexpected disconnect with a `ReconnectPolicy`, the transport enters a
+`RECONNECTING` state and calls `attemptReconnect()` which retries with
+exponential backoff up to `maxRetries` times.
+
+### Wire Protocol
+
+Uses the `v2.bsatn.spacetimedb` WebSocket subprotocol. All messages are BSATN
+(Binary SpacetimeDB Algebraic Type Notation) — a tag-length-value encoding
+defined in `crates/client-api-messages/src/websocket/v2.rs`.
+
+**Server messages** are preceded by a compression byte:
+- `0x00` — uncompressed
+- `0x01` — Brotli
+- `0x02` — Gzip
+
+The SDK requests Gzip compression via the `compression=Gzip` query parameter.
+
+### Client Cache
+
+`ClientCache` maintains a map of `TableCache` instances, one per table. Each
+`TableCache` stores rows keyed by content (`ByteArrayWrapper`) with reference
+counting. This allows overlapping subscriptions to share rows without duplicates.
+
+Transaction updates produce `TableOperation` events (Insert, Delete, Update,
+EventInsert) which drive the `TableHandle` callback system.
+
+### Threading Model
+
+- `WebSocketTransport` runs on a `CoroutineScope(SupervisorJob() + Dispatchers.Default)`.
+- All `handleMessage` processing is serialized behind a `Mutex` to prevent
+  concurrent cache mutation.
+- `atomicfu` atomics are used for transport-level flags (`idle`, `wantPong`,
+  `intentionalDisconnect`) that are read/written across coroutines.
+
+### Platform-Specific Code
+
+Uses Kotlin `expect`/`actual` for decompression:
+
+| Platform | Gzip | Brotli |
+|----------|------|--------|
+| JVM | `java.util.zip.GZIPInputStream` | `org.brotli.dec.BrotliInputStream` |
+| iOS | `platform.zlib` (wbits=31) | Not supported (SDK defaults to Gzip) |
+
+## Building
+
+```bash
+# Run all JVM tests
+./gradlew jvmTest
+
+# Compile JVM
+./gradlew compileKotlinJvm
+
+# Compile iOS (verifies expect/actual)
+./gradlew compileKotlinIosArm64
+
+# All targets
+./gradlew build
+```
+
+## Test Suite
+
+| File | Coverage |
+|------|----------|
+| `BsatnTest.kt` | Reader/Writer round-trips for all primitive types |
+| `ProtocolTest.kt` | ServerMessage and ClientMessage encode/decode |
+| `ClientCacheTest.kt` | Cache operations, ref counting, transaction updates |
+| `OneOffQueryTest.kt` | OneOffQueryResult decode (Ok and Err variants) |
+| `CompressionTest.kt` | Gzip round-trip, empty/large payloads (JVM only) |
+| `ReconnectPolicyTest.kt` | Backoff calculation, parameter validation |
+
+## Design Decisions
+
+1. **Manual ping/pong** instead of Ktor's `pingIntervalMillis` — OkHttp engine
+   doesn't support Ktor's built-in ping, so we implement idle detection
+   ourselves (matching the Rust SDK's 30s pattern).
+
+2. **ByteArray row storage** — Rows are stored as raw BSATN bytes rather than
+   deserialized objects. This keeps the core SDK schema-agnostic; code
+   generation (future) will layer typed access on top.
+
+3. **Compression negotiation** — The SDK advertises `compression=Gzip` in the
+   connection URI. Brotli is supported on JVM but not iOS; Gzip provides
+   universal coverage.
+
+4. **No Brotli on iOS** — Apple's Compression framework supports Brotli
+   (`COMPRESSION_BROTLI`) but it's not directly available via Kotlin/Native's
+   `platform.compression` interop. Since the SDK requests Gzip, this is a
+   non-issue in practice.

sdks/kotlin/README.md

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+# SpacetimeDB Kotlin SDK
+
+## Overview
+
+The Kotlin Multiplatform (KMP) client SDK for [SpacetimeDB](https://spacetimedb.com). Targets **JVM** and **iOS** (arm64, simulator-arm64, x64), enabling native SpacetimeDB clients from Kotlin, Java, and Swift (via KMP interop).
+
+## Features
+
+- BSATN binary protocol (`v2.bsatn.spacetimedb`)
+- Subscriptions with SQL query support
+- One-off queries (suspend and callback variants)
+- Reducer invocation with result callbacks
+- Automatic reconnection with exponential backoff
+- Ping/pong keep-alive (30s idle timeout)
+- Gzip and Brotli message decompression
+- Client-side row cache with ref-counted rows
+
+## Quick Start
+
+```kotlin
+val conn = DbConnection.builder()
+    .withUri("ws://localhost:3000")
+    .withModuleName("my_module")
+    .onConnect { conn, identity, token ->
+        println("Connected as $identity")
+
+        // Subscribe to table changes
+        conn.subscriptionBuilder()
+            .onApplied { println("Subscription active") }
+            .subscribe("SELECT * FROM users")
+
+        // Observe a table
+        conn.table("users").onInsert { row ->
+            println("New user row: ${row.size} bytes")
+        }
+    }
+    .onDisconnect { _, error ->
+        println("Disconnected: ${error?.message ?: "clean"}")
+    }
+    .build()
+```
+
+## Installation
+
+Add to your `build.gradle.kts`:
+
+```kotlin
+kotlin {
+    sourceSets {
+        commonMain.dependencies {
+            implementation("com.clockworklabs:spacetimedb-sdk:0.1.0")
+        }
+    }
+}
+```
+
+## Documentation
+
+For the SpacetimeDB platform documentation, see [spacetimedb.com/docs](https://spacetimedb.com/docs).
+
+## Internal Developer Documentation
+
+See [`DEVELOP.md`](./DEVELOP.md).

sdks/kotlin/build.gradle.kts

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+plugins {
+    kotlin("multiplatform") version "2.1.0"
+}
+
+group = "com.clockworklabs"
+version = "0.1.0"
+
+kotlin {
+    jvm()
+    iosArm64()
+    iosSimulatorArm64()
+    iosX64()
+
+    applyDefaultHierarchyTemplate()
+
+    sourceSets {
+        commonMain.dependencies {
+            implementation("io.ktor:ktor-client-core:3.0.3")
+            implementation("io.ktor:ktor-client-websockets:3.0.3")
+            implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.9.0")
+            implementation("org.jetbrains.kotlinx:atomicfu:0.23.2")
+        }
+        commonTest.dependencies {
+            implementation(kotlin("test"))
+            implementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:1.9.0")
+        }
+        jvmMain.dependencies {
+            implementation("io.ktor:ktor-client-okhttp:3.0.3")
+            implementation("org.brotli:dec:0.1.2")
+        }
+        iosMain.dependencies {
+            implementation("io.ktor:ktor-client-darwin:3.0.3")
+        }
+    }
+}
+
+tasks.withType<Test> {
+    testLogging {
+        showStandardStreams = true
+    }
+    maxHeapSize = "1g"
+}

sdks/kotlin/gradle.properties

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+kotlin.code.style=official
+kotlin.mpp.stability.nowarn=true
+org.gradle.jvmargs=-Xmx2g

sdks/kotlin/gradle/wrapper/gradle-wrapper.properties

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+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.12-bin.zip
+networkTimeout=10000
+validateDistributionUrl=true
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists