[executorch][native] Add native runtime Program reader + Graphviz visualizer

[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 C++ runtime components for the native backend, under
backends/native/runtime/:

• ptn::Program (Program.h / Program.cpp): a standalone reader that parses
  and verifies a serialized native graph (*.nptg). Program::load() copies
  the bytes, checks the NPTG FlatBuffer file identifier, runs the FlatBuffer
  verifier, and exposes a zero-copy view of the root; it throws
  std::runtime_error on failure. No ExecuTorch or torch dependency
  (flatbuffers only); exceptions allowed, C++11/17.
• Program::to_graphviz() (utils/ToGraphViz.cpp): renders the program to
  Graphviz DOT text — methods as clusters, dataflow / def-use as labeled edges,
  tensor metadata, constants, alias / mutation facts, quant, and HOP subgraphs.
  Pure string builder, no I/O. Doubles as the reference walk of
  native_backend::Program.

Also wires a --dot flag into the native_executor tool: load + verify via the
reader, then print the DOT to stdout.

The schema header is generated from serialization/native_graph.fbs via a
flatc genrule (:native_graph_schema). Mirrored into both the fbcode/ and
xplat/ trees to match the native backend layout.

Authored with Claude Code.

Differential Revision: D114396768

[pytorch-bot]

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