Let the mouse nodes look through a camera (#304)

- Behaviors (Mouse) and Concepts explain the new camera input of
  Mouse position and Is hovered.
- Architecture names pointerThrough.
- Development counts 1730 e2e checks.

Architecture.md

@@ -174,7 +174,7 @@ src/viewport/          WorldViewport (the drawing of a world or a holder, the ba
                        a `ViewSource` — the project and word of a change — so a released game draws without the
                        editor, #266; a view through a camera keeps only what that camera `Sees` and lays one
                        see-through view per overlay camera over itself, each handed its view and frame and
-                       drawn like any other, #302), gamePointer (the mouse handed to a running world), projectFiles (the file
+                       drawn like any other, #302), gamePointer (the mouse handed to a running world, in world units and on the main camera's picture, which `World.pointerThrough` turns into any other camera's view with `pointerThrough` in `core/properties/camera`, #304), projectFiles (the file
                        libraries pointed at the server, and what a project names fetched ahead),
                        fullscreen + FullscreenCorner (the browser's Fullscreen API, when the corner button shows,
                        and the button itself, shared by the Game tab and the released game, #267),

Behaviors.md

@@ -465,6 +465,14 @@ the inspector).
   `On mouse wheel` follows the same rule; what turned within one frame arrives as one `delta`,
   one notch of a mouse wheel being 1. A part a clip area cuts off is never hit, by neither of
   them nor by `Is hovered` (see [Concepts](Concepts)).
+  `Mouse position` and `Is hovered` take a **`camera`** (#304). Left open, they see the mouse
+  through the main camera as before. With a camera, they see it through that camera's picture as
+  an overlay camera lays it on, so a HUD button that only an overlay camera shows can be hovered.
+  `in view` then says whether the mouse is on that camera's picture; a camera that is not drawn
+  (switched off, or a second camera without *Overlay*) never has it. The position is kept on the
+  picture, so a main camera on the move does not drag the mouse across the HUD. `On click` and
+  `Mouse button` stay with the main camera. A HUD button is clicked with `pressed` of
+  `Mouse button` together with `Is hovered` through its camera.
 - **Idiomatic movement** (the demo's "Mover", five nodes): `On update → Move(Owner, dx = Axis
   input.x, dy = Axis input.y, scale = speed, dt = On update.dt)`, `speed` being a `Variables` node.
 - **Idiomatic platformer** (the example game's "Player"): `On update` adds `gravity · dt` to a

Concepts.md

@@ -54,13 +54,17 @@ the main camera at `z` 0. Something meant to lie *below* the world goes to a cam
 that is made the main one, with the game camera as an overlay. The main camera decides the size
 of the picture and the zoom of the tab; an overlay camera is fitted into that picture without
 distortion, so one of the same size and zoom covers it exactly. The mouse is reported through the
-main camera (#304 is about asking another). Two cameras switched on without *Overlay* draw as
+main camera; `Mouse position` and `Is hovered` ask another one through their `camera` input
+(#304, see *Mouse* in [Behaviors](Behaviors)). Two cameras switched on without *Overlay* draw as
 before: through the first one only. The Scene tab shows every layer. A camera can also wear a
 shader over its picture (#299, see *Wearing a shader* in [Shaders](Shaders)); it works on that
 camera's picture alone, before the pictures are laid on top of each other.
 Measured in the checks (step 63): a green sprite of an overlay camera at `z` -10 covers a red one
 of the main camera at `z` 1000, a `UI` sprite in front of the main camera and a `Default` one in
-front of the overlay stay away, and the same holds in the running game.
+front of the overlay stay away, and the same holds in the running game. With the overlay camera
+zoomed 2x, the real mouse 30 units right of the centre of the picture lies 15 units right of the
+overlay camera's centre (2015 at a camera on 2000): `Is hovered` through it finds the green
+sprite, through the main camera not.
 
 **A clip area cuts what is below it** (#260). It is an object of its own (*Clip area*, `+`
 menu) with a `width` x `height`, and it turns, scales and mirrors with its parent like a sprite.

Development.md

@@ -18,7 +18,7 @@ php tests/php/accounts.test.php                               # the API's accoun
                                                               #   has and the web server's container has not (SKIP there)
 php tests/php/snapshots.test.php                              # the snapshot store (#231), the same way
 php tests/php/releases.test.php                               # the release store (#266), the same way
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1729 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1730 editor checks, screenshots in /tmp/gm-e2e-*.png
 GM_STEPS=20 node tools/e2e.mjs                                # or 1-12, or 6,20 — a subset while working on one step
 ./e2e.sh start [GM_STEPS]                                     # the same run detached, see below
 ./e2e.sh status                                               # live progress, or the last run's result