feat: add finding model

[CoderDeltaLAN]

Adds a minimal dependency-free finding model for future diagnostics.

Scope:

• add Severity enum;
• add immutable Finding dataclass;
• add validation for required fields and optional location fields;
• add stable to_dict serialization for future console, JSON, and Markdown reporters;
• add unit tests for serialization and validation.

Not included:

• no CLI changes;
• no diagnostic engine;
• no discovery logic;
• no reporting formats;
• no CI changes.