[executorch][native] Add Value + index-arena ids to the in-memory IR

[SS-JIA]

Stack from ghstack (oldest at bottom):

• [executorch][native] Add Method + ValueRole to the in-memory IR #22281
• [executorch][native] Add Graph index arena to the in-memory IR #22280
• [executorch][native] Add Node to the in-memory IR #22279
• [executorch][native] Add Argument tagged union to the in-memory IR #22278
• -> [executorch][native] Add Value + index-arena ids to the in-memory IR #22277
• [executorch][native] Add Scalar value type to the in-memory IR #22276
• [executorch][native] Add TensorMeta + ScalarType in-memory IR types #22275
• [executorch][native] Add native runtime Program reader + DOT visualizer #22274

Next in-memory IR types for the native runtime, under
backends/native/runtime/graph/:

• Ids.h — NodeId / ValueId, plain int32_t index-arena aliases plus
  kInvalid, a valid() helper, and an
  in_bounds(id, size) helper that pairs the validity check with a range check
  via std::cmp_less, so neither side has to be cast to the other's signedness.
  NodeId indexes Graph.nodes; ValueId indexes Graph.values.
• Value (Value.{h,cpp}) — one SSA value (dataflow edge): a std::variant
  over std::monostate / TensorMeta / Scalar / std::vector<ValueId>, with
  the alternatives declared in ValueKind order so kind() is the variant's
  index. Plus def-use wiring (producer_id / consumer_ids), a storage
  alias_id, and an open attrs (std::any) scratch map. Construct via the
  per-kind constructors; read via typed accessors that throw on a kind mismatch.
  A List holds ValueIds to its element values (an arena grouping), so
  nesting is via the arena and there is no recursive value type; the AOT
  deserializer builds only Tensor / Scalar / None.

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: D114396769

[pytorch-bot]

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❌ 1 New Failure, 1 Unrelated Failure

As of commit 693d4ba with merge base a5f15b5 ():

NEW FAILURE - The following job has failed:

• pull / unittest-nxp-neutron / linux-job (gh)
  ../../dev/ir/converter/node_converter/test_mm_converter.py::TestMM::test__from_linear_without_bias__higher_ranks[PTQ-input_shape

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

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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