feat(kmp): base58 as first KMP module behind a shared-core XCFramework umbrella (#1201)

Establishes the umbrella model for cross-platform code sharing:

- kmp/shared-core becomes a thin aggregator that api-exports real KMP
  modules and publishes ONE SharedCore XCFramework (KMMBridge/SPM) so iOS
  consumes everything through a single import. It owns no logic itself.
- :libs:encryption:base58 becomes the first real KMP module (the pilot):
  pure Base58 in commonMain, JDK-only helpers (encodeChecked/decodeChecked/
  hashTwice/decodeToBigInteger) in androidMain, iOS targets via the default
  hierarchy template (no manual source-set wiring). shared-core re-exports it.
- The base58.json cross-platform vector gate now runs on BOTH the JVM and
  native targets (commonTest + a readResource expect/actual).
- All 9 Android consumers keep depending on :libs:encryption:base58
  unchanged — the KMP module still exposes the Android variant.
- Adds a flipcash.kmp.library convention plugin (registered; to be adopted
  by the next KMP modules).

Verified: base58 iOS compile, host vector gate, all 9 consumers + app
compile, and :kmp:shared-core:spmDevBuild produces SharedCore.xcframework.

Author: bmc08gt

Package.swift

@@ -0,0 +1,24 @@
+// swift-tools-version:5.9
+import PackageDescription
+
+let packageName = "SharedCore"
+
+let package = Package(
+    name: packageName,
+    platforms: [
+        .iOS(.v15)
+    ],
+    products: [
+        .library(
+            name: packageName,
+            targets: [packageName]
+        ),
+    ],
+    targets: [
+        .binaryTarget(
+            name: packageName,
+            path: "./kmp/shared-core/build/XCFrameworks/debug/\(packageName).xcframework"
+        )
+        ,
+    ]
+)

apps/flipcash/app/build.gradle.kts

@@ -175,6 +175,9 @@ androidComponents {
 
 dependencies {
     implementation(fileTree(mapOf("dir" to "libs", "include" to listOf("*.jar"))))
+    // KMP shared core — consumed transitively via :libs:encryption:base58 but declared
+    // explicitly so the app's own code can reference com.flipcash.shared.* directly.
+    implementation(project(":kmp:shared-core"))
     implementation(project(":services:flipcash-compose"))
 
     implementation(project(":apps:flipcash:core"))

build-logic/convention/build.gradle.kts

@@ -41,5 +41,9 @@ gradlePlugin {
             id = "flipcash.android.ed25519.shadow"
             implementationClass = "AndroidEd25519ShadowConventionPlugin"
         }
+        register("kmpLibrary") {
+            id = "flipcash.kmp.library"
+            implementationClass = "KmpLibraryConventionPlugin"
+        }
     }
 }

build-logic/convention/src/main/kotlin/KmpLibraryConventionPlugin.kt

@@ -0,0 +1,30 @@
+import org.gradle.api.Plugin
+import org.gradle.api.Project
+
+/**
+ * Convention plugin for KMP library modules.
+ *
+ * Applies:
+ *   - `org.jetbrains.kotlin.multiplatform`
+ *   - `com.android.kotlin.multiplatform.library`
+ *
+ * After applying this plugin the `kotlin {}` DSL block is available for configuring targets,
+ * source sets, and dependencies.
+ *
+ * Usage in a module's `build.gradle.kts`:
+ * ```
+ * plugins {
+ *     alias(libs.plugins.flipcash.kmp.library)
+ * }
+ * ```
+ */
+class KmpLibraryConventionPlugin : Plugin<Project> {
+    override fun apply(target: Project) {
+        with(target) {
+            with(pluginManager) {
+                apply("org.jetbrains.kotlin.multiplatform")
+                apply("com.android.kotlin.multiplatform.library")
+            }
+        }
+    }
+}

build.gradle.kts

@@ -32,6 +32,10 @@ plugins {
     alias(libs.plugins.androidx.room) apply false
     alias(libs.plugins.screenshot) apply false
     alias(libs.plugins.kover)
+    // KMP modules: shared-core umbrella + libs/* pilots
+    alias(libs.plugins.kotlin.multiplatform) apply false
+    alias(libs.plugins.android.kotlin.multiplatform.library) apply false
+    alias(libs.plugins.kmmbridge) apply false
 }
 
 // NOTE: The per-project `allprojects { }` configuration that used to live here

gradle.properties

@@ -48,3 +48,5 @@ android.enableR8.fullMode=true
 
 # Enabled parallel sync for Gradle 9.4+
 org.gradle.tooling.parallel=true
+
+kotlin.native.ignoreDisabledTargets=true

gradle/libs.versions.toml

@@ -5,13 +5,14 @@ android-targetSdk = "37"
 android-java = "21"
 
 kotlin = "2.4.10"
-ksp = "2.3.11"
+ksp = "2.3.10"
 kotlinx-coroutines = "1.11.0"
 kotlinx-serialization = "1.11.0"
 kotlinx-datetime = "0.8.0"
 
 kover = "0.9.9"
 agp = "9.3.1"
+kmmbridge = "1.2.1"
 google-services = "4.5.0"
 
 androidx-appcompat = "1.7.1"
@@ -48,26 +49,26 @@ hilt-jetpack = "1.4.0"
 okhttp = "5.4.0"
 retrofit = "3.0.0"
 kin-sdk = "2.1.2"
-grpc-android = "1.83.1"
+grpc-android = "1.83.0"
 slf4j = "1.7.36"
-firebase-bom = "34.17.0"
+firebase-bom = "34.16.0"
 play-service-ml-barcode = "18.3.1"
 play-services-auth-api-phone = "18.3.1"
 google-play-billing = "9.1.0"
 google-play-updates = "2.1.0"
 
-grpc = "1.83.1"
-grpc-okhttp = "1.83.1"
+grpc = "1.83.0"
+grpc-okhttp = "1.83.0"
 grpc-kotlin = "1.5.0"
 protobuf = "4.35.1"
 protobuf-plugin = "0.10.0"
 protovalidate-kt = "0.1.1"
 
-lib-phone-number-port = "9.0.36"
-lib-phone-number-google = "9.0.36"
+lib-phone-number-port = "9.0.34"
+lib-phone-number-google = "9.0.35"
 zxing = "3.5.4"
 
-androidx-benchmark-macro = "1.5.0-beta01"
+androidx-benchmark-macro = "1.5.0-alpha07"
 androidx-profileinstaller = "1.4.1"
 androidx-test-runner = "1.7.0"
 junit = "4.13.2"
@@ -325,6 +326,7 @@ flipcash-android-library = { id = "flipcash.android.library" }
 flipcash-android-library-compose = { id = "flipcash.android.library.compose" }
 flipcash-android-feature = { id = "flipcash.android.feature" }
 flipcash-android-ed25519-shadow = { id = "flipcash.android.ed25519.shadow" }
+flipcash-kmp-library = { id = "flipcash.kmp.library" }
 android-application = { id = "com.android.application", version.ref = "agp" }
 android-library = { id = "com.android.library", version.ref = "agp" }
 kotlin-android = { id = "org.jetbrains.kotlin.android", version.ref = "kotlin" }
@@ -344,3 +346,6 @@ androidx-baselineprofile = { id = "androidx.baselineprofile", version.ref = "and
 screenshot = { id = "com.android.compose.screenshot", version.ref = "screenshot" }
 kover = { id = "org.jetbrains.kotlinx.kover", version.ref = "kover" }
 protobuf-validate = { id = "dev.bmcreations.protovalidate", version.ref = "protovalidate-kt" }
+kotlin-multiplatform = { id = "org.jetbrains.kotlin.multiplatform", version.ref = "kotlin" }
+android-kotlin-multiplatform-library = { id = "com.android.kotlin.multiplatform.library", version.ref = "agp" }
+kmmbridge = { id = "co.touchlab.kmmbridge.github", version.ref = "kmmbridge" }

kmp/shared-core/build.gradle.kts

@@ -0,0 +1,41 @@
+plugins {
+    kotlin("multiplatform")
+    id("com.android.kotlin.multiplatform.library")
+    id("co.touchlab.kmmbridge.github") version "1.2.1"
+    `maven-publish`
+}
+
+group = "com.flipcash"
+version = "0.1.0"
+
+kotlin {
+    android {
+        namespace = "com.flipcash.shared"
+        compileSdk = 37
+        minSdk = 29
+    }
+    
+    val appleTargets = listOf(iosArm64(), iosSimulatorArm64(), iosX64())
+    appleTargets.forEach {
+        it.binaries.framework {
+            baseName = "SharedCore"
+            isStatic = true
+            export(project(":libs:encryption:base58"))
+        }
+    }
+
+    sourceSets {
+        commonMain {
+            dependencies {
+                api(project(":libs:encryption:base58"))
+            }
+        }
+    }
+}
+
+kmmbridge {
+    gitHubReleaseArtifacts()
+    spm(swiftToolVersion = "5.9") {
+        iOS { v("15") }
+    }
+}

kmp/shared-core/src/commonMain/kotlin/com/flipcash/shared/SharedCore.kt

@@ -0,0 +1,10 @@
+package com.flipcash.shared
+
+/**
+ * Marker for the cross-platform shared core. Gives iOS a symbol to verify the
+ * XCFramework is linked (`SharedCore.version`) and a home for the shared surface
+ * as it grows across milestones.
+ */
+object SharedCore {
+    const val version: String = "0.1.0"
+}

libs/encryption/base58/build.gradle.kts

@@ -1,12 +1,32 @@
 plugins {
-    alias(libs.plugins.flipcash.android.library)
+    kotlin("multiplatform")
+    id("com.android.kotlin.multiplatform.library")
 }
 
-android {
-    namespace = "${Gradle.codeNamespace}.encryption.base58"
-}
+kotlin {
+    android {
+        namespace = "com.getcode.encryption.base58"
+        compileSdk = 37
+        minSdk = 29
+        withHostTest {}
+    }
+    
+    iosArm64()
+    iosSimulatorArm64()
+    iosX64()
 
-dependencies {
-    testImplementation(kotlin("test"))
-    testImplementation(libs.kotlinx.serialization.json) // parse cross-platform test-vector fixtures
+    sourceSets {
+        commonMain {
+            // Pure Kotlin -- no external dependencies needed.
+        }
+        androidMain {
+            // MessageDigest + BigInteger -- JDK only; no extra Gradle deps.
+        }
+        commonTest {
+            dependencies {
+                implementation(kotlin("test"))
+                implementation(libs.kotlinx.serialization.json)
+            }
+        }
+    }
 }