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

[SS-JIA]

Stack from ghstack (oldest at bottom):

• [executorch][native] Add Argument tagged union to the in-memory IR #22235
• [executorch][native] Add Value + index-arena ids to the in-memory IR #22221
• [executorch][native] Add Scalar value type to the in-memory IR #22220
• -> [executorch][native] Add TensorMeta + ScalarType in-memory IR types #22219
• [executorch][native] Add native runtime Program reader + DOT visualizer #22218

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)). 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.

Dim's range constructor rejects a range no shape can have -- a negative lower
bound, or a bounded upper bound below it -- so a malformed serialized shape
fails where it enters the IR instead of surfacing later as a wrong numel().
numel() itself checks each product against INT64_MAX before multiplying,
since signed overflow is UB and an overflowed count would silently plan a
smaller buffer than the shape asks for.

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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❌ 1 New Failure, 3 Unrelated Failures

As of commit 038de22 with merge base cf9bcd8 ():

NEW FAILURE - The following job has failed:

• Build Presets / apple (ios-simulator) / build (gh)
  pytorch_tokenizers

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-editable / windows / windows-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following jobs failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / android / build-android (gh) (trunk failure)
• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (trunk failure)

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