Describe the pixel perfect camera switch

- Concepts: new paragraph Pixel perfect after Game view scale. It
  covers the grid per device pixel, Fit and the Scene tab, odd sizes
  and camera shaders.
- Development: e2e total 1742.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Concepts.md

@@ -104,6 +104,17 @@ Fixed steps start on a whole screen pixel, so pixel art at 100% stays sharp. The
 pixel, and Fit then names the factor in screen pixels (`Fit · 156%`). The choice is saved with
 the project, like *Play starts*, but it is no undo step: Ctrl+Z undoes edits, not a look at the game.
 
+**Pixel perfect**, a switch on the camera, off by default. On, the game view draws on its own pixel
+grid: one step per screen pixel a world unit gets, so at 100% on whole units and at 500% on fifths.
+The world's origin lands on a whole screen pixel, and every sprite, text and tilemap on a step of
+the grid, however the camera moves. Positions keep their fractions: only where things are drawn
+snaps, so a camera that follows smoothly still moves smoothly in the model. An unturned object
+snaps its top left corner, so a 15 px wide sprite is as sharp as a 16 px one; a turned one snaps
+its centre. Under a camera shader the grid is the shader picture's: with *Camera pixels* one step
+per camera pixel. The switch does nothing at **Fit**, where a camera pixel is no whole number of
+screen pixels anyway, and nothing in the Scene tab, where one works with the fractions. Particles
+are drawn where they are. An overlay camera has a switch of its own.
+
 **Fullscreen** (#267). The button at the right end of the Game tab's header puts the game view on
 its own into fullscreen: no toolbar, no panels, no tab header, no console, at the scale the picker
 holds. A run goes on as before and the keys still go to the game. A button in the top right corner

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                 # 1741 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1742 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