#3121 Docs: grant file reference shows a stale format and omits resources and subjects

Problem

content/manual/reference/swamp-serve/authorization.md shows a file-based grant example in a format swamp does not parse: a top-level YAML list with allow: and on: keys. A grant file is an object with a grants: list, and each entry takes subject, effect, actions, resource or resources, and optional condition and methods. The manual also does not document the resources array, nor the subjects array added by swamp-club#3070.

What to change

• Replace the method-scoping grant file example in reference/swamp-serve/authorization.md with the real format (grants: list; subject, effect, actions, resource, methods).
• Add a grant file reference (or extend the how-to how-to/swamp-serve/manage-access-grants.md) covering every entry field: subject or subjects (exactly one), effect, actions, resource or resources (exactly one; each array up to 100 items), condition, methods.
• Explain expansion: subjects and resources expand at parse time into one grant per subject and resource pair; reloading a file rewritten from repeated single-subject entries into one subjects entry changes no grants.
• Note when to prefer group: subjects (membership managed via swamp access group) over a subjects list kept in git.

Source of truth: design/enablers/access-control.md, section Grant files, in the swamp repo.