docs: evaluate migrating from Gradle to the Kotlin Toolchain

[rocketraman]

Adds docs/kotlin-toolchain-evaluation.adoc, the write-up of an evaluation of moving bootable's build from Gradle to Kotlin Toolchain 0.13.0 (formerly Amper).

Summary

• Verdict: feasible, with two project-local plugins. A scratch conversion builds all modules, passes the TestBalloon tests, and runs the wiring examples.
• Signing. The built-in signer needs an exported private key in an env var, so it can't use a YubiKey behind gpg-agent. A prototype plugin signs a staged publication with gpg (tested locally with a throwaway key; nothing uploaded).
• GraalVM native image. No built-in support. A prototype plugin runs native-image with the reachability metadata repository applied the way native-build-tools does; the server-ktor executable serves requests and shuts down cleanly on SIGTERM.
• Not blockers. The tag-derived version needs a one-line YAML rewrite in the release script, and the placeholder javadoc jar is an upstream design choice that Maven Central accepts.
• Upstream feedback. Findings were shared in the Kotlin Slack #kotlin-toolchain channel; the doc lists the seven YouTrack issues the maintainers linked, plus their notes on Dokka and compiler plugin versions.
• DI flavors. The toolchain needs one module per artifact. The doc records the decision to keep the flavors as source sets while on Gradle, and to split them into nested modules only if we migrate.

Notes for review

• Docs only; no build or code changes.
• The spike ran against the module set before the boot-db-* modules landed; module counts in the flavors section have been updated for current master.
• The spike's configuration and both plugins' source live only in the appendix; the scratch project itself isn't committed.
• Side finding: the Gradle server-ktor native smoke test on master currently builds a shared library (--shared, server-ktor.so) instead of an executable.

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.