Add Class authoring/parity: Class runtime model, IR support, and tests

[haydnv]

Motivation

• Provide a canonical v2-style user-defined Class authoring surface to mirror Python v1 model semantics while emitting versioned /class/{publisher}/{resource}/{version} definitions.
• Ensure library compilation and installation include language-neutral Class definitions and that bound Class methods construct a single deferred plan.

Description

• Add py/tinychain/classdef.py implementing Class, ClassError subclasses, class_definition, and validate_class_definition with canonical identity, prototype extraction, override validation, construction, and route binding.
• Extend py/tinychain/library.py to compile and include classes in the IR via compile_ir, adjust install to submit IR when classes are present, and adapt route compilation to handle class-bound routes.
• Update py/tinychain/__init__.py to export the new Class API and errors, and add State to py/tinychain/state/__init__.py exports.
• Add documentation py/CLASS_PARITY.md describing Class v1→v2 parity and usage, update py/README.md to reference it, and include a language-neutral fixture py/tests/fixtures/class_definitions.json.
• Add unit tests py/tests/test_class_authoring.py exercising class definition encoding, construction, bound methods, inheritance semantics, validation errors, manifest compilation, and equality with the golden fixture.

Testing

• Ran the new unit test file py/tests/test_class_authoring.py under pytest and the tests passed.
• Verified compile_ir yields an IR containing a classes list that matches class_definition outputs for declared Class types using the class_definitions.json fixture.
• Executed library install paths that exercise both legacy library_definition and new IR submission behavior and confirmed no regressions in route compilation.

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Codex Task