Eliminate duplicated constants across 15+ locations by introducing
spec/src/counts.rs as the single source of truth for all numeric
parameters, Ontology::enum_class_names() for the enum class list,
and env!("CARGO_PKG_VERSION") for automatic version propagation.

- New spec/src/counts.rs with all inventory constants
- New Ontology::enum_class_names() centralizes 13 enum class names
- Version auto-propagates from Cargo.toml via env!() macro
- 7 new {@count:} directive keys for docs content files
- Dynamic namespace count in website renderer
- All hardcoded counts in tests/validators replaced with counts:: imports
- Fixes version drift (was showing 5.0.0 instead of 5.1.0)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>