fix: add 16KB page alignment support for Android

[Obehi234]

Summary

Google Play now requires 16KB page alignment (0x4000) for native libraries targeting Android 15+ (API 35+). The current precompiled binaries shipped via Cargokit are built with 4KB alignment (0x1000), causing Play Store rejection.

This PR disables precompiled binary downloads to force source builds, which produce correct 16KB alignment when using NDK r28+.

Closes #181

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Investigation & Root Cause

How Cargokit Resolves Binaries

Cargokit computes a SHA-256 hash of the Rust source files (src/*.rs, Cargo.toml, Cargo.lock, cargokit.yaml) and uses it to look up precompiled binaries from the url_prefix in cargokit.yaml. If a matching binary is found, it's downloaded and used directly. Source compilation only happens as a fallback.

The Problem

The precompiled binaries hosted at https://github.com/LtbLightning/bdk-flutter/releases/download/precompiled_ were built with an older NDK that defaults to 4KB page alignment. We verified this by downloading the aarch64-linux-android release artifact and inspecting it:

$ llvm-readelf -l libbdk_flutter.so

Program Headers:
  Type   Offset   VirtAddr   PhysAddr   FileSiz  MemSiz   Flg  Align
  LOAD   0x000000 0x00000000 0x00000000 0x...    0x...    R    0x1000  ← 4KB
  LOAD   ...                                              R E  0x1000  ← 4KB

Why .cargo/config.toml Alone Doesn't Fix It

During Flutter builds, Cargokit sets CARGO_ENCODED_RUSTFLAGS in android_environment.dart. This environment variable overrides all other rustflags sources, including .cargo/config.toml. So adding linker flags there only helps standalone cargo build invocations, not Flutter builds through Cargokit.

The actual 16KB alignment in Flutter builds comes from NDK r28+ defaulting to it in its linker.

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Changes

File	Change
rust/cargokit.yaml	Commented out precompiled_binaries to force source builds
rust/.cargo/config.toml	Added explicit 16KB linker flags for all Android targets (safety net for standalone cargo builds)

Verification

Build Method	LOAD Alignment	Play Store
Precompiled binary (current)	0x1000 (4KB)	Rejected
Source build, NDK r28+	0x4000 (16KB)	Accepted

Tested end-to-end: built a Flutter app depending on bdk-flutter with this change, extracted libbdk_flutter.so from the APK, and confirmed 0x4000 alignment via llvm-readelf -l.

Tradeoffs

• Build time increases — source compilation instead of downloading precompiled binaries
• Recommended permanent fix — rebuild and republish precompiled binaries using NDK r28+, then re-enable the precompiled_binaries section. The config is commented out (not removed) to make this easy.