[plainprogrammer]

Closes #258. Spec 079 (spec, verification).

What it does

A Check deck link in the deck page's head opens /decks/:id/check: the same page with a "Deck check" section between the head and the zones. It is advisory and on demand. Nothing is stored, nothing is exported, and no finding blocks an edit.

• Deck size. Standard, Pioneer, Modern, Legacy, Vintage and Pauper: Main at least 60, Sideboard at most 15. Commander: Commander and Main together exactly 100.
• Copy limits. 4 of a card (1 in Commander), counted by card across printings and zones, owned plus proxy. Vintage restricted cards: 1. Basic lands and "any number" cards are exempt; a stated number ("up to seven") is held to that number in every format. Considering is never counted.
• Owned plus proxy. A card with owned copies and a proxy in one zone is flagged, for every deck including Casual. This is where a forgotten proxy shows up.
• A deck with no rules (Casual, or a format no longer registered) gets "Casual has no rules to check." and still gets the owned-plus-proxy result.

How

• Catalog::DeckChecks is the collectible-agnostic core (a third optional answer on the collecting vocabulary, after deck_card and deck_shape). Its header comment is the extension contract. It names no collectible, format or zone key (spec/deck_core_spec.rb).
• MTG::DeckRules and MTG::Collecting.deck_rules hold Magic's rules, read from the printings the page already loaded.
• Decks::ChecksController#show is a read-only GET (no form, no redirect). The results page carries turbo-cache-control: no-cache, so Back asks for a fresh check and never shows results from before an edit.
• The check adds no database statement beyond the plain deck page (13 for a 5-row and a 500-row deck alike); an example proves it and fails when a per-row query is added.
• New design-system pattern DeckCheck (c-deckcheck), documented in docs/design-system/components/DeckCheck.md.

Evidence

Screenshots are from headless Firefox with factory data and placeholder printings (no card art). Phone shots are a 360px viewport frame.

	Light	Dark
Results, desktop
Results, phone
Vintage, a restricted card
Commander, short, phone
Casual, no rules, phone
Plain page head, phone (three buttons)

Walkthrough (plain page, Check deck, results, remove a proxy, plain page again):

Verification

• Spec reviewed twice and the plan once, each read-only on Opus. Implementation review (Opus, Mode B): SPEC-ALIGNED; every AC from 1.1 to 7.6 has a proving example, listed in verification.md with the mutation proofs that show the examples can fail.
• bin/ci passed on 8c64f8ff (4,614 examples, 0 failures, 1 pending: the existing art-agreement spec that needs COLLECTOR_ART_AGREEMENT). Later commits are documents only.
• Scryfall's wording for the "any number" and "up to N" cards, and that name is the English name for a Japanese printing, were checked against api.scryfall.com and are quoted in verification.md.
• No migration, no export, import or backup change; export format stays at version 4.

Things to look at

• Deviations from the plan (all in verification.md): the component's classes are c-deckcheck because c-check is Form's checkbox-row class; a zone-head wrapping rule that was added and then dropped as out of scope; a plan expectation that was wrong in one example.
• Cosmetic, left: the focus ring on the "Deck check" heading spans the full content width on a direct visit.
• Pre-existing, not touched: a 60-character zone name with no spaces overflows the zone head on the plain deck page (no registered format has one).
• Housekeeping noticed, not done here: docs/architecture/README.md's "Specs it covers" cell for collection and lots stops at 073.
• One existing system example (menu_tab_bar_clearance_spec.rb:334, scroll restored from Turbo's cache) failed once in the pre-push bin/ci and passed on every rerun, alone and with the failing seed; it looks like a timing flake.

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