feat: publish prebuilt native assets

[HamdaanAliQuatil]

This builds trusted desktop native libraries during the pub.dev release workflow and includes them in the published package without committing binaries to the repository.

What changes:

• hook/build.dart uses a matching packaged library from prebuilt/<target>/, falling back to the existing CMake source build when no prebuilt is available.
• Consumers and CI can set build_from_source: true to bypass an available prebuilt explicitly.
• The publish workflow builds Linux x64, macOS arm64/x64, and Windows x64 artifacts, validates them, assembles the package, and publishes only from a version tag.
• Linux is built on Ubuntu 20.04 and checked for a maximum GLIBC 2.31 requirement. macOS deployment targets are checked at 10.15 for x64 and 11.0 for arm64.
• Pull requests run the same four-platform build and package validation. The validation exercises both the prebuilt and forced source-build paths and reports the exact compressed pub archive size.

Local validation:

• dart analyze hook/build.dart
• dart format --output=none --set-exit-if-changed hook/build.dart
• dart test -p vm test/aes_ctr_counter_wrap_test.dart

CI assembled the four-platform pub archive at 6.34 MiB compressed (6,646,685 bytes). The four native libraries are 8.61 MiB uncompressed (9,032,224 bytes).

[mosuem]

Hi @HamdaanAliQuatil , would you mind taking a look at #325 ? I implemented basically the same thing, based on https://github.com/unicode-org/icu4x/blob/main/ffi/dart/hook/build.dart, which works quite well.