Exclude internal artifacts from crates.io package

Trim the published package from 64 → 52 files (865.7KiB → 715.3KiB,
202.8KiB → 153.0KiB compressed) by excluding internal-only files:
.opencode/ agent configs, docs/reviews/, docs/research/, and
docs/sdd_process.md. Keeps docs/architecture/ ADRs and OQs, which
document the public design.

Verification:
- cargo test --release: 396 tests pass
- cargo clippy --all-targets -- -D warnings: clean
- cargo doc --no-deps: clean
- cargo build --target wasm32-unknown-unknown --release: clean
- cargo publish --dry-run --allow-dirty: clean (52 files, 153.0KiB)

Cargo.toml

@@ -9,6 +9,7 @@ repository = "https://git.alk.dev/alkdev/alktype"
 readme = "README.md"
 keywords = ["binary", "jsonschema", "wire-format", "serialization", "layout"]
 categories = ["encoding", "data-structures", "parsing"]
+exclude = [".opencode/", "docs/reviews/", "docs/research/", "docs/sdd_process.md", "Cargo.lock"]
 
 [lib]
 name = "alktype"