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>

lang/src/runtime/numeric/intarray_core_box.hako

@@ -0,0 +1,28 @@
+// IntArrayCoreBox — thin Hako wrapper over NyRT IntArrayCore (handle-based)
+// This provides a box-level API for numeric kernels while delegating storage
+// to nyash.intarray.* extern calls in NyRT (ring1).
+
+static box IntArrayCore {
+  init { handle, len }
+
+  static new(len) {
+    local h = externcall "nyash.intarray.new_h"(len)
+    local b = new IntArrayCore()
+    b.handle = h
+    b.len = len
+    return b
+  }
+
+  length() {
+    return externcall "nyash.intarray.len_h"(me.handle)
+  }
+
+  get_unchecked(idx) {
+    return externcall "nyash.intarray.get_hi"(me.handle, idx)
+  }
+
+  set_unchecked(idx, v) {
+    externcall "nyash.intarray.set_hii"(me.handle, idx, v)
+    return null
+  }
+}

lang/src/runtime/numeric/mat_i64_box.hako

@@ -0,0 +1,65 @@
+// MatI64 — simple i64 matrix box built on top of IntArrayCore.
+// Internal layout: rows, cols, stride, core (IntArrayCore).
+
+using nyash.core.numeric.intarray as IntArrayCore
+
+static box MatI64 {
+  init { rows, cols, stride, core }
+
+  static new(rows, cols) {
+    local total = rows * cols
+    local core = IntArrayCore.new(total)
+    local m = new MatI64()
+    m.rows = rows
+    m.cols = cols
+    m.stride = cols
+    m.core = core
+    return m
+  }
+
+  rowsCount() {
+    return me.rows
+  }
+
+  colsCount() {
+    return me.cols
+  }
+
+  at(r, c) {
+    local idx = r * me.stride + c
+    return me.core.get_unchecked(idx)
+  }
+
+  set(r, c, v) {
+    local idx = r * me.stride + c
+    me.core.set_unchecked(idx, v)
+    return null
+  }
+
+  // Naive O(n^3) matmul: this * b
+  mul_naive(b) {
+    local n = me.rows
+    local mcols = me.cols
+    local bcols = b.cols
+    // assume shapes are compatible and square for now (Phase 21.6 draft)
+    local out = MatI64.new(n, bcols)
+    local i = 0
+    loop(i < n) {
+      local k = 0
+      loop(k < mcols) {
+        local aik = me.at(i, k)
+        local j = 0
+        loop(j < bcols) {
+          local idx = i * out.stride + j
+          local v = out.core.get_unchecked(idx) + aik * b.at(k, j)
+          out.core.set_unchecked(idx, v)
+          j = j + 1
+        }
+        k = k + 1
+      }
+      i = i + 1
+    }
+    return out
+  }
+}
+