Build Rust-first Zygisk guest runtime

[Zhanfg]

Implements the first executable milestone of the Rust-first Zygisk framework on an isolated feature branch.

Current scope:

• installable arm64 Magisk/KernelSU-style module ZIP;
• public Zygisk API v4 guest module;
• minimal C++ lifecycle and FD adapter only;
• Rust process policy retaining only com.android.providers.downloads;
• versioned framed Rust companion protocol;
• Rust root companion forwarding;
• persistent Rust daemon and rzctl status CLI;
• fail-open fallback state file;
• 16 KiB-compatible Android linker page-size settings;
• host fmt, Clippy and unit tests plus Android arm64 build CI.

This PR intentionally does not install hooks or modify specialization arguments. Provider-mode Zygote monitoring and injection will follow after Guest mode is validated on-device.

[Zhanfg]

Guest-mode milestone v0.1.0 now builds successfully in GitHub Actions.

Validated:

• rustfmt normalization: pass
• Clippy with -D warnings: pass
• unit tests: pass
• Android arm64 cross-compile: pass
• Rust staticlib + minimal C++ Zygisk ABI link: pass
• module ZIP integrity: pass
• exported entries: zygisk_module_entry, zygisk_companion_entry
• ELF target: Android API 26 / arm64-v8a
• load-segment alignment: 0x4000 (16 KiB compatible)
• final Zygisk library stripped

Release ZIP SHA-256:
fdee4f245a5ed84ab067030ec7ad1a2d6cdf61057fb2850b6ea7135a09d54b50

This remains a no-hook Guest preview: it only verifies provider loading, pre-specialize companion FD preservation, post-specialize Rust IPC, daemon registration and CLI diagnostics for com.android.providers.downloads.