Include inherited slots when hashing slotted objects

[Kenoia]

DeepHash iterates obj.__slots__, which can omit parent-defined slots and split a string declaration into characters. Different slot values can then produce equal hashes, causing ignore_order=True to miss changes.

• Collect declared slots across the MRO and treat a string declaration as one slot
• Keep the existing uninitialized-slot and __getattr__ behavior
• Add 11 regression/control cases for inheritance, declaration forms, fallback behavior and ignored type groups

Related: #506 and #577

Validation:

• Full pytest suite, including slow tests: 1320 passed, 8 skipped (Python 3.13)
• All 11 added regression and control cases pass
• Pyright and the CI flake8 error gate passed