Particles: the preview stands beside the graph and wears the asset's own rows (#86)

A hold moves the field's origin instead of pausing a counter, which is the other
side of "a particle is a function of its age" and now stated in Architecture.
Development gains the rule that a step opens a tab with openTab(), not by
clicking its name, and both counters go to 816.

Architecture.md

@@ -79,8 +79,8 @@ src/editor/texture/    TextureEditor (header + picker), TextureCanvas (picture +
                        ImagePicker + imageGrid (the "+ → Texture" dialog), checker (the transparency board)
 src/editor/animation/  AnimationEditor (header + picker), AnimationCanvas (preview + frame strip), strip (its geometry)
 src/editor/statemachine/ StateMachineEditor (header + picker, on the graph canvas)
-src/editor/particles/  ParticlesEditor (graph canvas above a preview strip, splitter between them),
-                       ParticlePreview (the strip: the asset's own particles at the type's defaults)
+src/editor/particles/  ParticlesEditor (graph canvas left of a preview panel, splitter between them),
+                       ParticlePreview (the panel: the asset's own particles, wearing its own rows)
 src/editor/list/       ListEditor (a row per item), slot (which list is being edited, and how it is written back)
 src/editor/console/    log (ring buffer, argument formatting, the console.* capture), ConsolePanel (the tail under the tabs)
 api/                   PHP backend: index.php (routing), lib/{Store,TextureStore,FontStore,OriginalName,Response}.php
@@ -102,7 +102,10 @@ outline, a name plate or a checkerboard left outside it is drawn over the panel
 per frame** — `core/particles/emission.ts` says which particles are alive at second *t* and
 `core/particles/compile.ts` turns a graph into what one of them looks like at its age, so a field
 of 500 particles is reproducible, holds still for a check, and needs no state beyond where its
-source stood when each one was born. Both are pure and tested without a browser.
+source stood when each one was born. Both are pure and tested without a browser. The other side of
+that choice: **freezing such a thing means moving its origin, not pausing a counter** — the clock
+belongs to the library and keeps running, so a preview let go after a minute on hold has to be
+rebased to the second it was stopped at, or it resumes a minute further on.
 Comments state contracts
 and reasons, not what the code does: a `/** */` on every export whose name does not say it all,
 `//` only for a decision or gotcha.

Development.md

@@ -12,7 +12,7 @@ npm run check        # typecheck + lint + format:check + tests, the gate CI runs
                      #   code is lost (tail's is reported), so a red run reads as green
 npm run build        # writes dist/ and regenerates index.html
 ./build.sh           # the same under `set -o pipefail`, for callers that pipe the output
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 809 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 816 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
@@ -68,6 +68,11 @@ scene state and share the CDP driver in `tools/lib/harness.mjs` — `press`, `un
 `object(id)`, `world()`, `mutate(js)` and friends, so a check reads like the interaction it
 exercises.
 
+A step that needs a tab opens it with `openTab(t, 'container')` from `tools/e2e/lib/scaffold.mjs`,
+not by clicking its name: since the tenth tab the row hides some of them at the test window's width,
+and the helper goes through the `»` menu when it has to. Clicking the label directly took the
+whole suite down the day the Particles tab was added.
+
 `GM_STEPS=20`, `GM_STEPS=1-12` or `GM_STEPS=6,20` runs a subset — seconds instead of minutes
 while one step is being written. Because a step lives off what the earlier ones left behind, a
 subset that skips a prerequisite fails on purpose; take `1-N` when in doubt.
@@ -151,4 +156,4 @@ widths to find click points. The **render device is not in this repo** — `/dev
 and a 1 GB `/dev/shm` come from `container.options` in the runner's own `config.yaml`, which
 applies to every job on that runner. Without them chromium falls back to llvmpipe, which gets
 WebGL 1.0 only. Measured in the job container: `ANGLE (Intel, Vulkan 1.3.230, HD Graphics 630)`,
-WebGL 2.0, and the same 809 checks as here.
+WebGL 2.0, and the same 816 checks as here.

Particles.md

@@ -9,7 +9,7 @@ the model was chosen:
 
 - **The same second always looks the same.** Two runs at the same frame rate, or a different one,
   draw the same picture — the field is a function of the clock, not of how many frames have gone by.
-- **Any second can be shown at once.** The preview strip in the tab has a slider: drag it to 2 s and
+- **Any second can be shown at once.** The preview panel in the tab has a slider: drag it to 2 s and
   the particles stand where they are at 2 s, exactly, with nothing simulated on the way there.
 - **Collision is out, for good.** "Sparks bounce off the ground", wind, drag mid-flight — none of
   these can be written as a function of age, and no later addition will change that.
@@ -17,7 +17,7 @@ the model was chosen:
 ## The Particles tab
 
 `+` in the **Assets** panel makes a particle asset and opens the **Particles** tab on it (a
-double-click on an existing one does the same; a click only selects). The upper part is the same
+double-click on an existing one does the same; a click only selects). The left part is the same
 canvas the Behavior and Shader tabs have: right-click places a node, drag moves, drag from a port
 links, `Snap` snaps to the 24 px grid, `Fit` frames the graph, undo takes every step back. The
 readout on the right says `N parameters` while the graph compiles, otherwise why it does not
@@ -25,11 +25,18 @@ readout on the right says `N parameters` while the graph compiles, otherwise why
 compiling shows the reason on its body and a red frame; a source whose graph does not compile emits
 nothing.
 
-Below it, the **preview** strip emits the asset's own particles from its middle, with the type's
-default rows (20 a second, one second each, a plain 16 px square). `Hold` stops the clock where it
-is and turns into `Run`; the slider next to it jumps to any second in the first three lifetimes;
-`Restart` begins the stream again at particle 0. The strip deliberately draws plain squares: which
-picture a source throws belongs to the placed object, not to the asset the tab is showing.
+Right of it, past a divider that drags, the **preview** panel emits the asset's own particles from
+its middle, wearing **the rows the asset itself carries** — edit `Rate` or `Lifetime` on the asset
+and the panel changes with them, including how far the slider reaches (three lifetimes). Rows the
+asset leaves alone fall back to the type's defaults: 20 a second, one second each, a plain 16 px
+square. It stands beside the graph rather than under it because a graph is read across: width is
+what it can spare, height is not. `Hold` stops the clock where it is and turns into `Run`; the
+slider jumps to any second in the first three lifetimes — and holds the panel too, so the button
+reads `Run` after a drag as well; `Restart` begins the stream again at particle 0, which a held
+panel shows as 0.00 s rather than keeping the second it stood on. **A hold owes nothing when it
+ends**: letting the panel run again carries on from the second it was stopped at, however long it
+stood there — the field's origin is moved, rather than the library clock being allowed to run on
+underneath the freeze.
 
 ## The nodes
 
@@ -105,8 +112,8 @@ to a cold one over `Age`.
 - **A source is culled by nothing.** Its particles may be anywhere on screen, so the source itself is
   never skipped for being out of frame — with many sources in one scene that costs.
 - **Particle time is the editor's clock, not the game's.** Pausing the game does not freeze a spray.
-- **The preview shows the asset, not an instance**: the type's default rows and a plain square, never
-  the texture or the rate a particular placed source was given.
+- **The preview shows the asset, not an instance**: the rows the asset carries, never the ones a
+  particular placed source was given — every one of them may be overridden where it is placed.
 - **A source with `Emitting` off draws nothing at all** once the last particle has died, so it
   disappears from the scene view; it is still in the hierarchy panel, and selecting it there shows
   its handle.