[executorch][native] Add TensorMeta + ScalarType in-memory IR types

[SS-JIA]

Stack from ghstack (oldest at bottom):

• [executorch][native] Add deserializer bridge (Program -> Method) #22105
• [executorch][native] Add Method + ValueRole to the in-memory IR #22104
• [executorch][native] Add Graph index arena to the in-memory IR #22103
• [executorch][native] Add Node to the in-memory IR #22102
• [executorch][native] Add Argument tagged union to the in-memory IR #22101
• [executorch][native] Add Value + index-arena ids to the in-memory IR #22100
• [executorch][native] Add Scalar value type to the in-memory IR #22099
• -> [executorch][native] Add TensorMeta + ScalarType in-memory IR types #22098
• [executorch][native] Add native runtime Program reader + Graphviz visualizer #22097
• [executorch][native] Add native_executor test harness #22096

First concrete in-memory IR value types for the native runtime, under
backends/native/runtime/graph/:

• ScalarType (ScalarType.h) — a standalone, header-only scalar element-type
  enum driven by an X-macro table (PTN_FORALL_SCALAR_TYPES(cpp_type, name, id)), modeled on the ET-VK vkapi::ScalarType. The macro generates the enum
  (ids pinned to ExecuTorch's ScalarType / the native_graph.fbs schema so a
  deserializer maps the serialized byte directly), the k<Name> constants, the
  forward ScalarType -> C++ type trait (ScalarTypeToCppType<N> /
  cpp_type_t<N>), and the element_size() / scalar_type_name() helpers.
  Half / BFloat16 map to uint16_t as a raw storage stand-in.
• TensorMeta (TensorMeta.{h,cpp}) — logical tensor metadata: element type,
  per-dim size ranges (Dim{min, max}), and an advisory dim_order_hint
  (non-prescriptive — engines choose their own physical layout). Helpers:
  ndim(), is_static(), is_contiguous(), numel() (upper-bound extent;
  throws on an unbounded dim), to_string(), and equality.

Pure std — no ExecuTorch and no flatbuffers dependency. Mirrored into both the
fbcode/ and xplat/ trees to match the native backend layout.

Authored with Claude Code.

Differential Revision: D114396765

[pytorch-bot]

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