PyClass has no decorators field — class decorators are silently dropped #127

Problem

PyClass (codeanalyzer/schema/py_schema.py) has no decorators field. PyCallable
has one (py_schema.py:274, filled at syntactic_analysis/symbol_table_builder.py:334),
but the class branch of the builder never reads ClassDef.decorator_list, so every class
decorator is dropped silently — no field, no warning.

Verified on main (6f02581) against a fixture with @dataclass class Account:

CLASS 'app.Account'  (has 'decorators' key? False)
  method 'doubled'  decorators=['property']

The methods carry theirs; the class carries nothing. This loses @dataclass,
@app.errorhandler, Django/pydantic class decorators, and @pytest.mark.* on test
classes — the decorators that classify what a class is.

Scope boundary

Adds decorators: List[str] to PyClass and populates it, mirroring how PyCallable
already works. Does not resolve decorator strings to ids (#128), does not
touch call-graph edges (#129), and does not add any new node type.

Goals

• PyClass.decorators: List[str] = [], unparsed source strings in source order
• Populated for top-level and nested/inner classes
• Neo4j: reuse the existing :PyDecorator node and PY_DECORATED_BY relationship,
  widening its start label from ["PyCallable"] to include PyClass
  (neo4j/schema.py:237), plus the flat decorators: string[] property
• Bump Neo4j SCHEMA_VERSION (additive)

Caveats and known risks

• Schema v2 change, so it enters at designing-cldk-changes, not a patch. It is
  purely additive (new optional field, defaults to []), so the monotonicity gate and
  every existing consumer are unaffected.
• Sibling analyzers share the vocabulary. TypeScript has class decorators too;
  whatever name and shape lands here should be the canonical one, not a Python rename.
• Available from L1 (it is symbol-table data), so it must not be gated behind a level.

Definition of done

• The fixture above emits decorators: ["dataclass"] on app.Account
• An inner class with a decorator emits it too
• A class with no decorators emits [], not a missing key
• analysis.json(-a 1) ⊆ ... ⊆ (-a 4) still holds
• Neo4j PY_DECORATED_BY traverses from a :PyClass node