[executorch][native] Add Scalar value type to the in-memory IR

[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

Add Scalar (backends/native/runtime/graph/Scalar.{h,cpp}), a concrete scalar
value — an int / double / bool tagged over a trivially-copyable union, analogous
to c10::Scalar and the scalar slice of the ET-VK ComputeGraph Value.

Construct implicitly from int / int64_t / double / bool; read via strict
accessors (to_int / to_double / to_bool, which throw on a tag mismatch) or
the promoting to<T>() template (a static_cast of whichever member is active,
like c10::Scalar::to<T>()). Plus tag() / is_*() predicates and tag_name()
/ to_string() debug helpers. No "none" state — the graph's Value owns that.

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: D114396762

[pytorch-bot]

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❌ 1 New Failure

As of commit 96f437d with merge base 9a2d135 ():

NEW FAILURE - The following job has failed:

• pull / unittest / macos / macos-job (gh)
  torchaudio

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