refactor(miniapp): move builtin contracts to product domains

[limityan]

Summary

• Move built-in MiniApp bundle, install marker, content hash, source payload, placeholder, package payload, and seed-decision contracts into bitfun-product-domains::miniapp::builtin.
• Keep core as the owner of bundled asset includes, user-data filesystem IO, marker read/write, customization metadata IO, and recompile orchestration.
• Extend decomposition docs, local AGENTS guidance, and boundary checks so this pure contract move is not mistaken for runtime/IO migration.

Functional Boundary

• Behavior-preserving refactor only; no MiniApp asset content, seed/update logic, storage layout, worker runtime, host dispatch, or product surface behavior changes.
• bitfun-product-domains gains sha2 only behind the miniapp feature. --no-default-features remains limited to serde / serde_json.

Validation

• git fetch --no-tags git@github.com:GCWing/BitFun.git main:refs/remotes/gcwing/main
• git rebase gcwing/main
• git diff --check gcwing/main...HEAD
• node scripts/check-core-boundaries.mjs
• cargo tree -p bitfun-product-domains --no-default-features --depth 1 --edges features
• cargo test -p bitfun-product-domains --no-default-features
• cargo test -p bitfun-product-domains --features miniapp
• cargo test -p bitfun-product-domains --features product-full
• cargo test -p bitfun-core miniapp::builtin::tests:: -- --nocapture
• cargo check -p bitfun-core --features product-full