Remove duplicate Kotlin build module lib/

[Copilot]

Two parallel Kotlin build modules existed (kotlin/lib/ and kotlin/walletkit/) serving the same purpose, with lib/ being an outdated copy never wired into the Gradle build.

Changes

• Deleted kotlin/lib/ — the entire directory including build.gradle.kts, consumer-rules.pro, .gitignore, and src/main/jniLibs/.gitignore

The lib/ module was never included in settings.gradle.kts, not referenced by any CI workflow or build script, and used stale settings (Java 1.8, version resolved via GitHub API curl) compared to the canonical kotlin/walletkit/ module (Java 17, version read from Cargo.toml).

No changes to kotlin/walletkit/, settings.gradle.kts, or any build scripts — they already pointed exclusively to walletkit/.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Remove duplicate kotlin build scripts</issue_title>
<issue_description>There are two kotlin build scripts. We should unify to a single one.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Remove duplicate kotlin build scripts #222

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