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

[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

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

• Ids.h — NodeRef / ValueRef, plain int32_t index-arena aliases plus
  kInvalid, a valid() helper, and an
  in_bounds(ref, 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.
  NodeRef indexes Graph.nodes; ValueRef indexes Graph.values.
• Value (Value.{h,cpp}) — one SSA value (dataflow edge): a std::variant
  over std::monostate / TensorMeta / Scalar / std::vector<ValueRef>, with
  the alternatives declared in ValueKind order so kind() is the variant's
  index. Plus def-use wiring (producer_ref / consumer_refs), a storage
  alias_ref, 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 ValueRefs 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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✅ You can merge normally! (2 Unrelated Failures)

As of commit 441f497 with merge base cf9bcd8 ():

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

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

BROKEN TRUNK - The following job 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)

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