docs: define derived inventory count guidance

[Teakowa]

Fixes #12

• Removes the duplicated source-of-truth obligation for hand-maintained mutable inventory counts in durable documentation.
• Makes the catalog, registry, manifest, dataset, or adjacent list the inventory owner; retained derived displays must use a deterministic generation/validation path.
• Distinguishes contractual and illustrative numbers, and routes test/fixture/corpus counts to Ban test and fixture counts from durable documentation #11.
• Preserves numeric contracts and examples.