Contract 6.4.0

The population's decks against what one player holds (feedback #70, 2026-09-20). A corpus deck sorted by win rate reads as advice, and the payload carried nothing about the caller's collection: an agent recommended a deck the player could not field - two of the top four rows ran a card he did not own, the top row cost him two mean levels at his own card levels. Jamie's call: recommendations come from the player's collection, and the agent must be free to say what a few upgrades would open.

• battles_meta_decks takes fit_for (a player tag with a recorded collection). Every returned row's cards carry held_level; every row carries fit: fieldable, missing (each unowned card or locked form with its reason), own_mean_level (the deck at the player's levels), vs_fielded (against fit_for.fielded_mean_level, the mean level of the decks the player actually played in the window and mode), upgrades (each held card below the fielded level, largest deficit first, with the levels needed) and mean_level_after_upgrades. A row the player cannot field leaves decks[] for unfieldable[], after sort and limit, so the population's ranking is unchanged and an agent cannot recommend what is not in the array. A fit_for block says whose collection, as of when, and the benchmark. An unrecorded collection is refused (not_recorded) rather than read as owning nothing.
• battles_meta_cards takes fit_for too: each row carries held (level, forms_unlocked, has_form) or null when the card is not owned.
• Without fit_for, both tools open their notes with the sentence that the rows are the population's and check nothing about any one player, and that a recommendation to a person should pass fit_for. With it, the note says what mean_level_gap is on a meta row - the population's edge, never the caller's - since the same name means the caller's on battles_decks.
• players_collection carries fielded - { days: 30, mean_level, battles }, the mean card level of the decks the player actually played in the last thirty days - so 128 held levels have a benchmark without a second call.

Additive.