[executorch][native] Add Graph index arena 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

Adds ptn::Graph, the index arena that owns the value layer landed so far. It
is a pure function body (mirrors the schema Graph): it holds the Nodes and
Values that the NodeRef / ValueRef handles point into, the ordered graph
input / output value lists, and -- recursively -- the subgraph bodies for
higher-order ops. Stateful method-level bindings (constants, output specs,
mutable buffers) are deferred to a later Method type.

Design decisions baked in:

• Per-graph subgraph arena. Graph owns std::vector<Graph> subgraphs and
  GraphRef indexes the enclosing graph's subgraphs, matching the schema's
  recursion and the per-Graph SSA namespace so a subgraph stays self-contained
  with its parent. (A std::vector<Graph> member of Graph is legal C++17 --
  the standard containers permit an incomplete value type at the point of the
  member declaration.)

• Storage identity vs execution order are decoupled. nodes is an
  append-only arena so a NodeRef never shifts (the index-arena invariant),
  while schedule (std::vector<NodeRef>) carries the topological / execution
  order a runtime walks. At load the arena order equals the wire's topological
  order and schedule is the identity [0, n) (reset_schedule()); across
  mutation the arena order is no longer topological, so schedule is
  authoritative -- reorder / insert there (moving int32s, invalidating no ref)
  rather than moving storage. Deletion via tombstone + a compacting pass is
  deferred until a mutating pass needs it.

• Pure arena. inputs / outputs are ValueRef lists (schema SSA names
  resolved to refs at deserialize); no tensor_values side table in memory
  (each Value already carries its TensorMeta); the name to ref map stays
  deserializer-local.

rebuild_def_use() recomputes every Value's producer / consumers from the
nodes (order-independent -- it walks the arena, not schedule). Placeholder and
Output nodes are real entries in nodes, so def-use is uniform: a graph input
value's producer is its placeholder node, and graph inputs are identified by
membership in inputs, not by producer == kInvalid. This corrects the now
stale Value.h producer comment (also in this diff). Bounds-checked
node() / value() / subgraph() accessors throw on an invalid ref, and
to_string() gives a multi-line dump in schedule order.

Pure std only (no ExecuTorch, no flatbuffers), consistent with the rest of the
standalone ptn runtime.

Differential Revision: D114396767

[pytorch-bot]

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