feat: add prebuilt native asset infrastructure

[HamdaanAliQuatil]

This adds the first prebuilt-native-asset infrastructure without checking in any binary artifacts.

What changes:

• hook/build.dart now looks for a matching packaged prebuilt under prebuilt/<target>/ before falling back to the existing CMake source build.
• Adds a force_source_build user define so consumers/CI can explicitly bypass prebuilts and exercise the source-build path.
• Documents the expected prebuilt layout and binary policy in prebuilt/README.md.
• Adds a manual build-prebuilt-assets workflow that builds host prebuilts and uploads them as workflow artifacts. It does not commit binaries or open PRs.

The intended model is still conservative: arbitrary contributor PRs should not introduce binary blobs. Actual prebuilt binaries should come from trusted automation or another explicit release process.

Validation:

• dart analyze hook\\build.dart
• dart test -p vm test/aes_ctr_counter_wrap_test.dart
• Locally tested the prebuilt-selection path with a temporary copied Windows DLL, then removed the binary before committing.