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>

hako.toml

@@ -0,0 +1,4 @@
+[env]
+# This file mirrors nyash.toml for Hakorune/Hako tools.
+# Primary runtime config remains nyash.toml; hako.toml exists as a dev-facing alias.
+