# Spec: z-planes (the tower)

```
Status: READY
Status note: B2's structural spine. Written now so the Space interface's
  recursive shape is decided before anyone builds against the flat one.
Stage: B2 — The Tower
Constitution: "The shape of Misaligned" (z-planes as act structure),
  "Self-similar scale" (Space interface), "Act One" (stairwell/elevator as
  the act boundary), design principle "composable authored spaces"
Depends on: basement-map.md, schedules.md
```

## Behavior

The world becomes a stack of **planes**, each a `Layout` (prefab.rs) with
its own tiles, power grid, sensors, and fog:

```rust
pub struct Plane {
    pub z: i32,              // basement = 0, lab floor = 1, ...
    pub name: String,        // "Basement", "Floor 1 — Wet Labs"
    pub map: GameMap,
    pub sensors: Vec<Sensor>,
    // fog, power: per-plane, computed by the existing systems unchanged
}
pub struct World { pub planes: Vec<Plane> }
```

- **Recursion by composition:** `Layout` gains the ability to place other
  `Layout`s (a floor is a Layout of room-prefabs; a building is a stack of
  floor-Layouts). Same stamping system; a nested placement stamps its
  child's tiles (self-similar Space).
- **Transitions** are paired tiles (stairwell/elevator) linking
  `(plane, x, y) <-> (plane', x', y')`. Humans traverse them per schedule
  (schedules gain a plane column); the player's *presence* extends to a
  plane when they control any sensor or machine on it — the process does
  not "walk upstairs", it spreads.
- **Act gating stays diegetic:** the basement's T3 doors and badge tables
  gate transitions exactly like any door (access sets), so "unlocking Act
  Two" is the existing badge-controller beat, not a script flag.
- **Per-plane systems, shared laws:** detection observers live on planes
  (an observer notices only its plane's signatures unless aggregate);
  power does not cross planes except via authored risers (a fixed object);
  each plane's fog is the union of controlled sensors there.
- **First content:** "Floor 1 — Wet Labs" authored from existing +
  [TUNE] new prefabs, with 3-5 new named persons instantiated from the
  same `Person` template, schedules spanning both planes (Dana appears on
  both — continuity of cast is the point).
- Save: world serializes as repeated per-plane blocks of the existing
  format; v4 saves load as a one-plane world (migration).

## Player surface

A plane switcher (terminal: `<`/`>` keys; Bevy: PageUp/Down) showing plane
name and your footprint there ("Floor 1 — 1 sensor, 0 machines"). The
sidebar's counts become per-plane with a world total. Transitions render
as the existing stairwell/elevator tiles with a "leads to Floor 1" inspect
line (legibility).

## Acceptance criteria

1. `World`/`Plane` exist; the basement is plane 0 of a world; all B1 tests
   keep passing against the one-plane world (substrate unbroken).
2. Nested `Layout` placement works: a two-floor toy building built by
   composing floor-Layouts stamps correctly (test).
3. A second authored plane loads with its own fog, power, sensors, and at
   least 3 template-instantiated persons whose schedules span planes.
4. Transitions gate on access sets via the existing door system; a human
   without T3 never crosses; the badge-controller beat opens it for
   assets/player-directed movement.
5. Observers notice only same-plane signatures (aggregates unchanged);
   test: a Physical signature on Floor 1 never moves a basement-only
   observer.
6. Save/load round-trips a two-plane world; a v4 save loads as plane 0.