Add S4 — offline Android app for BIP-39 → SLIP-39 paper backup (加固&&破解) - #2

S4 — offline Android app for BIP-39 → SLIP-39 paper backup.

• What it does: Android app (com.s4, Kotlin, targetSdk 37) splitting any BIP-39 12/15/18/21/24-word seed or raw entropy into N SLIP-39 shares (2-16, threshold 1≤T≤N, default 6/any-3, 20-33 words each) via vendored bc-shamir/bc-slip39; restores exact seed from any T with SHA-256 fingerprint; provides verbatim Recovery Guide for decades-later restore via any SLIP-39 tool (iancoleman.io/slip39, Trezor, python-mnemonic). 100% offline (no INTERNET permission, FLAG_SECURE, GPL-3.0, 91 JVM + 35 instrumented tests).
• Why 加固&&破解 / Harden&&Crack: Fills empty Hardening section — device-level paper backup companion to dexknife-wj/apkprotect app hardening. 100% offline, no network.
• Source: https://github.com/heyaibi/s4 — [Kotlin], 1 star, 6d, 100% offline.

Section: 加固&&破解 / Harden&&Crack (Readme.md + Readme_en.md, format - [**1**星][6d] [Kotlin] [heyaibi/s4](url) description).