[calebl]

Stage 6 of the dungeon plan, server half ("The countdown and the collapse").
A floor that says minutes: gives way in a game that many story minutes
after that game first stood on it; a player on it then dies with it. The
collapse is the game's only: the world's rows are untouched (VISION.md
principle 7). The narrator is not told. A world without minutes plays as
before; no checked-in story world turns it on.

• Migration 20261031120000 AddMinutesToFloorsAndFellAtToPlaythroughFloors,
  the engine's version and declaration. schema.rb moves to 2026_10_31_120000.
• Floor validates minutes as a positive integer or nil.
  Playthrough::FloorReached#falls_at and #fallen?.
• Playthrough#fallen?(location) mirrors Game::fallen (the room's own floor
  row or its parent's, the first by id). #cast_in returns nobody on a fallen
  floor, so #foes_in follows; #exits leaves such rooms out, and
  Classifier#exits_here now reads #exits, so Mechanics::State follows.
• Loader reads floor minutes (a positive integer only; absent writes nil);
  the exporter writes it when set.
• The extension's glance_json, Playthrough::Glance (Floor, #floor,
  #collapse, console lines) and Protocol::V1#glance carry floor and
  collapse; docs/protocol/v1/openapi.json and v1.md add them as revision 1.7.
• The play page's end notice prints the collapse sentence on its own line
  before the death statement, which is unchanged.
• Story::Doctor: floor_fallen_with_no_countdown (manual, never repaired).
• SqlitePackage already copies playthrough_floors column for column; its
  test now pins fell_at.
• Sweep: world a-floor-that-falls, scripts waiting-out-a-floor and
  a-floor-falls-behind-you, goldens vendored and checks files written.

Against the engine PR's list

renderedstep/engine#98 lists 11 items under "What the server pin PR must change". This PR covers all of them, with three differences:

• Item 2. The model is Playthrough::FloorReached (table playthrough_floors), the name stage 5 gave it, not Playthrough::Floor.
• Item 5. Mechanics::State reads exits through Playthrough::Classifier#exits_here. That method used to read Location#exits directly, so it now reads Playthrough#exits.
• Item 10, the world dump. parity/worlds/a-floor-that-falls.sql belongs to the engine repo. I built it from this branch's loader as the engine's parity README says, in a scratch export of this commit. Its rows match the engine PR's hand-made dump exactly. The only differences are the position of the schema_migrations row and ar_internal_metadata.schema_sha1, which come from loading the schema rather than migrating. The engine PR can keep its dump or take this one.

Gates (both provider keys unset, no model calls)

• bin/rails test: 5474 runs, 0 failures, 0 errors (includes rake game:sweep)
• bin/rails engine:rust_gates: PASSED, 112 scripts
• bin/rails engine:vendored: VENDORED at 064084f
• bundle exec rubocop: no offenses. bin/rails zeitwerk:check: all good. bin/brakeman: no warnings.

Once renderedstep/engine#98 merges, this moves to its merge commit.