Adopt sk.ainet.multiplatform + sk.ainet.npm-pins from SKaiNET-build-logic - #5

…ogic

Replaces every module's hand-rolled plugins{}/kotlin{ android{...};
jvm(); androidNativeArm32(); ...; js{browser()}/wasmJs{browser()} }
boilerplate (copy-pasted 6 times, one line different per module) with
id("sk.ainet.multiplatform") + a per-module skainet { namespace = ... }
block and a gradle.properties skainet.targets declaration (must be a
Gradle property, not part of the skainet{} DSL -- Kotlin targets have to
exist before the module's own kotlin{} block runs, see SkainetTargets'
own doc comment).

Supersedes #3 (the earlier manual linuxX64 fix): the "linux" target
group expands to linuxX64+linuxArm64 together, so this exact class of
bug -- one target silently missing from a hand-copied list -- is now
structurally impossible instead of caught after the fact.

sk.ainet.npm-pins applied at root (real application, not apply false):
every module here builds js and/or wasmJs targets, and
sk.ainet.multiplatform refuses to configure a web target unless it's in
force at the root. No pins declared yet (nothing to pin currently) --
this just puts the mechanism in place for when one's needed.

Both plugin versions pinned once via pluginManagement.plugins{} in
settings.gradle.kts rather than at each application site: they share an
implementation jar, and Gradle can't cleanly resolve more than one
plugin ID from the same jar with an explicit version at each individual
site ("plugin is already on the classpath with an unknown version").

kotlin-test in commonTest is now automatic (skainet {
kotlinTestInCommonTest } defaults true) -- the explicit
implementation(libs.kotlin.test) lines every module had are gone,
nothing else changed about test dependencies.

Verified: full multi-project configuration green; ./gradlew
publishToMavenLocal (728 tasks, all 6 modules x every target) green;
the repo's own validate-published-poms.sh gate passes clean on all 70
published POMs (group/version resolution for internal project()
dependencies unchanged); jvmTest + linuxX64Test green.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>