docs: plan the card list implementation

Eleven tasks against the card-list spec, TDD throughout, each with its own
mutation check.

Three decisions the plan makes that the spec did not:

  - Task 0 rebases the branch first. item-20-message-rows is 15 commits behind
    master and conflicts in eight files, one of which carries the item 55
    splitter fix in the same file this plan rewrites heavily. Resolving that
    while also building a feature would mean debugging a merge and a feature at
    once, so it is its own task with its own verification step.
  - CardLayout is a separate file from CardDelegate, holding all the geometry
    and touching no painter or widget. That is a direct response to the
    rendering-probe lessons in CLAUDE.md: a delegate test can be defeated by a
    blank render, a pure function cannot.
  - Tasks 2, 5 and 6 knowingly leave the build red between commits, since
    removing a column across three files cannot be atomic without one enormous
    commit. The plan says so at each one and warns against merging mid-plan.

Also recorded: the account chip is specified but has no task, because its
placement on a card was never specified and inventing one would be a guess.

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