refactor(build-logic): extract the KMP test-fixture generator into a convention plugin

[bmc08gt]

Problem

libs/encryption/base58/build.gradle.kts and libs/codes/kikcode/build.gradle.kts each carried a byte-identical copy of GenerateTestFixtures — differing only in the package the generated file is declared in (com.getcode.vendor vs com.getcode.codes.kikcode) — plus an identical block wiring the AGP lint tasks to the generator:

tasks.matching { it.name.startsWith("lint") || it.name.endsWith("LintModel") }
.configureEach { dependsOn(generateTestFixtures) }

That copy-paste already broke code/cash once. #1288 added the lint wiring to kikcode; #1289 then introduced the same generator in base58 without it, and lint failed over an undeclared dependency on generated sources (fixed in #1292).

The failure mode is easy to hit: kotlin.srcDir(taskProvider) carries the task dependency to the Kotlin compile tasks only, while lint reads the same source directories straight off disk. Nothing about copying the generator suggests the extra wiring is needed, and it's invisible until CI runs lint.

Change

Move the task into build-logic behind a new flipcash.kmp.test.fixtures convention plugin:

• GenerateTestFixtures — the task, unchanged apart from the package becoming a @get:Input property. The output path is derived from it, so com.getcode.vendor still lands at com/getcode/vendor/TestFixtures.kt.
• TestFixturesExtension — packageName (required) and fixtures (defaults to src/commonTest/resources).
• KmpTestFixturesConventionPlugin — registers the task, adds the generated directory to commonTest, and wires the lint dependency automatically. A module that applies the plugin cannot omit it.

Both modules drop from ~75 lines of build script to four:

plugins {
kotlin("multiplatform")
id("com.android.kotlin.multiplatform.library")
alias(libs.plugins.flipcash.kmp.test.fixtures)
}
testFixtures {
packageName ="com.getcode.vendor"
}

Notes

packageName has no default, so a module that applies the plugin and forgets to set it fails with Gradle's standard "property 'packageName' doesn't have a configured value". I opted against defaulting to the Android namespace: base58's namespace (com.getcode.encryption.base58) doesn't match its fixture package (com.getcode.vendor), and a silently-wrong default is worse than a loud missing one.