feat(perf): add Phase 21.8 foundation for IntArrayCore/MatI64 numeric boxes

Prepare infrastructure for specialized numeric array benchmarking:
- Add IntArrayCore plugin stub (crates/nyash_kernel/src/plugin/intarray.rs)
- Add IntArrayCore/MatI64 box definitions (lang/src/runtime/numeric/)
- Add Phase 21.8 documentation and task tracking
- Update nyash.toml/hako.toml with numeric library configuration
- Extend microbench.sh for matmul_core benchmark case

Next: Resolve Stage-B MirBuilder to recognize MatI64/IntArrayCore as boxes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

crates/nyash_kernel/src/plugin/mod.rs

@@ -6,6 +6,7 @@ pub mod instance;
 pub mod invoke;
 pub mod invoke_core;
 pub mod map;
+pub mod intarray;
 pub mod semantics;
 pub mod string;
 
@@ -17,5 +18,6 @@ pub use instance::*;
 pub use invoke::*;
 pub use invoke_core::*;
 pub use map::*;
+pub use intarray::*;
 pub use semantics::*;
 pub use string::*;