[executorch][native] Add the .ptn package facade and its inspector

[SS-JIA]

Stack from ghstack (oldest at bottom):

• [executorch][native] Map .ptn packages instead of copying them #22532
• -> [executorch][native] Add the .ptn package facade and its inspector #22471
• [executorch][native] Add a safetensors index reader #22470
• [executorch][native] Add a zip reader for .ptn packages #22469
• [executorch][native] Add a minimal JSON reader for package metadata #22468
• [executorch][native] Add deserializer bridge (Program -> Method) #22467
• [executorch][native] Claim getitem in the partitioner #22466

The three readers below this each handle one layer of a .ptn. Package is the
facade that puts them together and answers the question the runtime actually
has: give me the program, and give me the bytes for constant key.

Package::load opens the archive, takes the program.ptg member as the
serialized program, parses program.safetensors when present, and reads
aliases.json when the exporter deduplicated byte-identical immutable
constants. constant(key) resolves an alias to its owner and reports which key
owns the bytes, so a caller can tell a shared weight from a private one.
owner_keys counts each blob once; keys lists everything resolvable.

It takes ownership of a moved buffer rather than copying, so a model-sized
package is resident once. Every span it hands out -- and the zip and safetensors
readers it holds -- aliases that one buffer. std::vector's move preserves the
buffer address, which is what makes the defaulted move constructor safe; copy is
deleted, because a package is model-sized.

A package with no constants is a normal case, not an error: tensors_ is a
std::optional and stays empty, since a program that references nothing has no
safetensors member at all.

ptn_inspector is the tool that exercises all of it. --constants lists every
key with dtype, shape, size and an FNV-1a fingerprint of its bytes, then
reconciles the listing against the methods' DataBindings in both directions --
bindings with no package key, and package keys no binding claims. Two behaviour
notes: --constants is a flag rather than taking a path, since the external
constant file it would have named does not exist in any current format; and the
default mode verifies the flatbuffer rather than only checking for NPTG at
offset 4, which previously reported success on a truncated program. A bare
.ptg is rejected here with a message pointing at ptg_inspector.

Differential Revision: D118480654

[pytorch-bot]

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❌ 1 New Failure

As of commit a3faec1 with merge base 2c1da32 ():

NEW FAILURE - The following job has failed:

• pull / test-samsung-models-linux / linux-job (gh)
  AttributeError: type object 'Test_Milestone_MobileBertFinetune' has no attribute 'model_cache_dir'

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