sandialabs / LoQS / 42140515486

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Fix serialization bugs that read back wrong data or failed to write (#171)

* Key the encode cache by a content digest

Make the encode cache's key a SHA-256 digest of each object's typed
content, so values whose Python hashes collide (-1 and -2; 1, True and
1.0) are no longer written as copies of each other and read back wrong.

- _serial_hash now returns the first 16 bytes of a SHA-256 digest of a
  type-tagged, self-delimiting stream, laid out in one commented table.
- The key includes a Serializable's class, keys cycles by depth rather
  than address, and raises on a type the encoders can't write.
- encode raises when ignore_no_serialize_flags is combined with an
  encode cache, since the key honours no-serialize flags.
- Minor: tests for colliding ints, mixed types, strings, lists against
  tuples and subclasses in JSON and HDF5, for a checkpointed shot that
  collides with a program object, and for the new guard.

Co-authored-by: claude-sonnet-5-5 <noreply@anthropic.com>
Co-authored-by: claude-opus-5-5 <noreply@anthropic.com>

* Make dict merges atomic per entry and store UTF-8 strings

Roll back a merge_dict_attr entry that raises, so later keys keep their
own values, and store HDF5 string datasets as exact UTF-8 or raw bytes,
so non-ASCII str and bytes lists round-trip instead of failing.

- Each merge_dict_attr entry is all or nothing: a failure trims both
  sides back and drops the encode-cache entries the entry added.
- A fresh attribute writes only its first entry, and one append loop
  does the rest; a failed first entry deletes the attribute.
- str datasets hold strict UTF-8 sized in bytes and bytes datasets hold
  the bytes as they are; a value that can't read back exactly raises.
- Appends measure width in bytes and refuse bytes on a str side, or the
  reverse, with TypeError.
- One decoder reads dataset sides, and the store's side helpers move
  into streamingmerge.py.
- Minor: tests for refused entries and retries, non-ASCII round tr... (continued)

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