docs(core): Layout lifecycle documentation

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+# UI Layout Lifecycle
+
+## Overview
+
+There can be at most one UI layout running. The running layout is stored in
+`ui.CURRENT_LAYOUT`. The value of this attribute must only be managed internally by the
+layout objects themselves.
+
+There are two kinds of layouts. The `Layout` class represents the normal kind of layout
+which can accept user interaction or timer events. Such layout can return a _result_
+of the interaction, retrievable from the `Layout.get_result()` async method. Typically,
+calling code will block on an `await` for the result.
+
+`ProgressLayout` represents loaders for long-running operations. It does not respond to
+events and cannot return a result. Calling code will start the progress layout in the
+background, call to it to update progress via `ProgressLayout.report()`, and then stop
+it when done.
+
+## Individual layout lifecycle
+
+A newly created layout object is in **READY** state. It does not accept events, has no
+background tasks, does not draw on screen.
+
+When started, it moves into **RUNNING** state. It is drawn on screen (with backlight
+on), accepts events, and runs background tasks. The value of `ui.CURRENT_LAYOUT` is set
+to the running layout object.
+
+(This implies that at most one layout can be in **RUNNING** state.)
+
+Layout in **RUNNING** state may stop and return a result, either in response to a user
+interaction event (touch, button click, USB) or an internal timer firing. This moves it
+into a **STOPPED** state. It is no longer shown on screen (backlight is off unless
+another layout turns it on again), does not accept events, and does not run background
+tasks.
+
+A layout in a **STOPPED** state has a **result** value, available for pickup by awaiting
+`get_result()`.
+
+Stopping a layout before returning a result, or retrieving a result of a **STOPPED**
+layout, will move it back to **READY** state.
+
+### State transitions
+
+```
+-------+    start()   +-----------+    <event>    +-----------+
+| READY | -----------> |  RUNNING  | ------------> |  STOPPED  |
+-------+              +-----------+               +-----------+
+  ^   ^                      |                           |
+  |   |                      |                           |
+  |   +------- stop() -------+                           |
+  |                                                      |
+  +--------------------- get_result() -------------------+
+```
+
+Calling `start()` checks if other layout is running, and if it is, stops it first. Then
+it performs the setup and moves layout into **RUNNING** state.
+
+(At most one layout can be in **RUNNING** state at one time. That means that before
+a layout moves to **RUNNING**, the previously running layout must move out.)
+
+When layout is in **RUNNING** state, calling `start()` is a no-op. When layout is in
+**STOPPED** state, calling `start()` fails an assertion.
+
+After `start()` returns, the layout is in **RUNNING** state. It will stay in this state
+until it returns a result, or is stopped.
+
+Calling `stop()` on a **READY** or **STOPPED** layout is a no-op. Calling `stop()` on a
+**RUNNING** layout will shut down any tasks waiting on the layout's result, and move to
+**READY** state.
+
+After `stop()` returns, the layout is not in **RUNNING** state and the current layout is
+no longer this layout.
+
+Awaiting `get_result()` will resume the lifecycle from its current stage, that is:
+
+* in **READY** state, starts the layout and waits for its result
+* in **RUNNING** state, waits for the result
+* in **STOPPED** state, returns the result
+
+After `get_result()` returns, the layout is in **READY** state.
+
+All state transitions are synchronous -- so, in terms of trezor-core's cooperative
+multitasking, effectively atomic.
+
+## Global layout lifecycle
+
+When Trezor boots, `ui.CURRENT_LAYOUT is None`. The screen backlight is on and displays
+the "filled lock" welcome screen with model name.
+
+When a layout is started, the backlight is turned on and the layout is drawn on screen.
+`ui.CURRENT_LAYOUT` is the instance of the layout.
+
+When a layout is stopped, the backlight is turned off and `ui.CURRENT_LAYOUT` is set to
+`None`.
+
+Between two different layouts, there is always an interval where backlight is off and
+the value of `ui.CURRENT_LAYOUT` is `None`. This state may not be visible from the
+outside; it is possible to synchronously go from `A -> None -> B`. However, there MUST
+be a `None` inbetween in all cases.
+
+## Button requests
+
+A `ButtonRequest` MUST be sent while the corresponding layout is already in **RUNNING**
+state. That is, in particular, the value of `ui.CURRENT_LAYOUT` is of the corresponding
+layout.
+
+The best choice is to always use the `interact()` function to take care of
+`ButtonRequest`s. Explicitly sending `ButtonRequest`s is not supported.
+
+## Debuglink
+
+We assume that only one caller is using the debuglink and that debuglink commands are
+strongly ordered on the caller side. On the firmware side, we impose strong ordering on
+the received debuglink calls based on the time of arrival.
+
+There are two layout-relevant debuglink commands.
+
+### `DebugLinkDecision`
+
+Caller can send a decision to the **RUNNING** layout. This injects an event into the
+layout. In response, the layout can move to a **STOPPED** state.
+
+A next debug command is read only after a `DebugLinkDecision` is fully processed. This
+means that:
+
+* if the decision caused the layout to stop, subsequent debug commands will be received
+  by the next layout up, and
+* if the decision did not cause the layout to stop, subsequent debug commands will be
+  received by the same layout.
+
+### `DebugLinkGetState`
+
+Caller can read the contents of the **RUNNING** layout.
+
+There are three available waiting behaviors:
+
+* `IMMEDIATE` (default) returns the contents of the layout that is currently
+  **RUNNING**, or empty response if no layout is running.
+* `NEXT_LAYOUT` waits for the layout to change before returning. If no layout is
+  running, waits until one is started and returns its contents. If a layout is running,
+  waits until it shuts down and a new one appears.
+* `CURRENT_LAYOUT` waits until a layout is running and returns its contents. If no
+  layout is running, the behavior is the same as `NEXT_LAYOUT`. If a layout is running,
+  the behavior is the same as `IMMEDIATE`.
+
+When received after a `ButtonRequest` has been sent, the modes guarantee the following:
+
+* `IMMEDIATE` and `CURRENT_LAYOUT`: return the contents of the layout corresponding to
+  the button request (unless the layout has already been terminated by a timer event or
+  user interaction, in which case the result is undefined).
+* `NEXT_LAYOUT`: waits until the layout corresponding to `ButtonRequest` changes.
+
+When received after a `DebugLinkDecision` has been received, the behavior is:
+
+* `IMMEDIATE`: If the layout did not shut down (e.g., when paginating), returns the
+  contents of the layout as modified by the decision. If the layout shut down, the
+  result is not guaranteed.
+* `CURRENT_LAYOUT`: Returns the layout that is the result of the decision.
+* `NEXT_LAYOUT`: No guarantees.
+
+While `DebugLinkGetState` is waiting, **no other debug commands are processed**. In
+particular, it is impossible to start waiting and then send a `DebugLinkDecision` to
+cause the layout to change. Doing so will result in a deadlock.
+
+(TODO it _might_ be possible to lift this restriction.)
+
+If a layout is shut down by a `DebugLinkDecision`, and the firmware expects more
+messages, a new layout might not come up until those messages are exchanged. Calling
+`DebugLinkGetState` except in `IMMEDIATE` mode will block the debuglink until the new
+layout comes up. If the calling code is waiting for a `DebugLinkGetState` to return, it
+will deadlock.
+
+(Firmware tries to detect the above condition and sends an error over debuglink if the
+wait state is `CURRENT_LAYOUT` and there is no current layout for more than 2 seconds.)
+
+## Synchronizing
+
+`ButtonRequest` is a synchronization event. After a `ButtonRequest` has been sent from
+firmware, all debug commands are guaranteed to hit the layout corresponding to the
+`ButtonRequest` (unless the layout is terminated by a timer event or user interaction).
+
+`DebugLinkDecision` is also a synchronization event. After a `DebugLinkDecision` has
+been received by the firmware, all debug commands are guaranteed to hit the layout
+that is the "result" of the decision.
+
+In order to synchronize on a homescreen, it is possible to either:
+
+* invoke any workflow that triggers a `ButtonRequest`, and follow it until end
+  (`Ping(button_protection=True)` would work fine), or
+* poll `DebugLinkGetState` until the layout is `Homescreen`. Typically, running
+  `DebugLinkGetState(wait_layout=CURRENT_LAYOUT)` will work on the first try if you are
+  close enough to homescreen (such as after completing a workflow).
+
+`wait_layout=NEXT_LAYOUT` _cannot_ be used for synchronization, because it always
+returns the _next_ layout. If the current one is already homescreen, it will wait
+forever.