[executorch][native] Add native_executor test harness

[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 a standalone C++ tool, native_executor, that loads a serialized native
graph (*.nptg) and its optional out-of-line constant file into memory and
reports what was loaded. Today it verifies the FlatBuffer NPTG file
identifier and holds the constant file as opaque bytes; it has no ExecuTorch or
torch dependency. This is the seed of a central harness that will grow
load / inspect / benchmark / validate / profile subcommands as the native
runtime lands.

Mirrored into both the fbcode/ and xplat/ trees to match the native backend
layout.

Authored with Claude Code.

Differential Revision: D114396766

[pytorch-bot]

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