Contract 9.15.0

Every elixir_timeline item carries id, the story it tells, and revision, how far that story has grown (#111), so an agent tells a story once, recognises it in the next read, and updates it when it grows rather than retelling it.

• id is opaque and the same across reads and windows. A ledger moment's comes from its ledger row (a duplicated row that the read serves once keeps the lowest row's); a sitting's from its player and its first battle, found by walking back past the window's start, so battle_session and session_standout of one sitting share it; the rest from what names the happening. It names the happening, not the reader: a member's moment has one id on the clan's timeline and on the player's own.
• revision is 1 for a moment that never grows. A sitting's is its battles counted from its first battle (a standout's, up to the last rung the window learned), so a sitting told at 20 battles and read again at 40 is the same id at a higher revision.
• The member-read note says to update on a higher revision where it said to keep the newest item per kind and started_at. /api/me/timeline carries the fields too.

Additive. The JSON API has no timeline operation and is unchanged.