feat: add SimConstants.ts and doc/model.md stubs

CLAUDE.md

@@ -11,6 +11,7 @@ Reusable single-screen SceneryStack template. Run `npm run rename` to fork it to
 | File | Purpose |
 |---|---|
 | `src/SimColors.ts` | All `ProfileColorProperty` instances |
+| `src/SimConstants.ts` | Named numeric constants (layout px, physics SI units) |
 | `src/SimNamespace.ts` | Namespace for color property names |
 | `src/i18n/StringManager.ts` | Singleton localized string accessor |
 | `src/sim-screen/SimScreen.ts` | Screen wrapper |

doc/model.md

@@ -0,0 +1,37 @@
+# Model - Sim Template
+
+This document describes the model (the underlying physics, math, and behavior) for the simulation, in
+terms appropriate for an educator. It is the companion to
+[implementation-notes.md](./implementation-notes.md), which targets developers. Replace the placeholder
+content below with a description of your own sim's model when you fork this template.
+
+## Overview
+
+*One or two paragraphs describing what the simulation models and the key ideas a student should take
+away. Write for a teacher, not a programmer — avoid code and class names.*
+
+The template ships with no domain-specific physics; it is a minimal Model-View scaffold. A real sim
+replaces this section with its actual conceptual model.
+
+## Quantities and units
+
+*List the primary modeled quantities, their symbols, units (SI where applicable), and ranges.*
+
+| Quantity | Symbol | Units | Range |
+|---|---|---|---|
+| *(example)* time | t | s | 0 – ∞ |
+
+## Governing equations
+
+*State the equations or rules that drive the model, with a sentence explaining each. Use a smaller
+fixed time step than the screen default if the integration requires it; the model makes no assumption
+about frame rate.*
+
+## Simplifications and assumptions
+
+*Note anything intentionally idealized or omitted relative to the real-world phenomenon (e.g. no air
+resistance, point masses, instantaneous response), so educators can set expectations.*
+
+## References
+
+*Optional: textbook sections, papers, or standards the model is based on.*

src/SimConstants.ts

@@ -0,0 +1,37 @@
+/**
+ * SimConstants.ts
+ *
+ * Central repository for every named numeric constant used across the
+ * simulation. Bare numbers that carry semantic meaning (sizes, margins,
+ * physics defaults, ranges) belong here rather than inline in model or view
+ * code, so they are named, documented, and changed in one place.
+ *
+ * Conventions
+ * ───────────
+ *  - Physics / model values use SI units (metres, seconds, kilograms, …);
+ *    note the unit in a comment on each value.
+ *  - Layout / chrome values are in screen pixels.
+ *  - Colour strings live in SimColors.ts, not here.
+ *  - Computed expressions (e.g. `2 * Math.PI`) may stay inline.
+ *
+ * Remove the example constants below and replace them with the sim's own.
+ */
+
+import SimNamespace from "./SimNamespace.js";
+
+// ── Layout / chrome (screen pixels) ───────────────────────────────────────────
+
+/** Margin between the screen edge and edge-anchored controls (e.g. Reset All). */
+export const SCREEN_VIEW_MARGIN = 20;
+
+/** Corner radius shared by control panels and dialogs. */
+export const PANEL_CORNER_RADIUS = 6;
+
+// ── Physics / model defaults (SI units) ───────────────────────────────────────
+
+// Example: export const GRAVITY_MPS2 = 9.81; // m/s²
+
+SimNamespace.register("SimConstants", {
+  SCREEN_VIEW_MARGIN,
+  PANEL_CORNER_RADIUS,
+});

src/common/SimPanel.ts

@@ -29,13 +29,14 @@ import type { Node } from "scenerystack/scenery";
 import type { PanelOptions } from "scenerystack/sun";
 import { Panel } from "scenerystack/sun";
 import SimColors from "../SimColors.js";
+import { PANEL_CORNER_RADIUS } from "../SimConstants.js";
 
 export class SimPanel extends Panel {
   public constructor(content: Node, providedOptions?: PanelOptions) {
     super(content, {
       fill: SimColors.panelBackgroundColorProperty,
       stroke: SimColors.panelBorderColorProperty,
-      cornerRadius: 6,
+      cornerRadius: PANEL_CORNER_RADIUS,
       xMargin: 12,
       yMargin: 10,
       ...providedOptions,

src/sim-screen/view/SimScreenView.ts

@@ -25,12 +25,10 @@ import { ResetAllButton } from "scenerystack/scenery-phet";
 import type { ScreenViewOptions } from "scenerystack/sim";
 import { ScreenView } from "scenerystack/sim";
 import SimColors from "../../SimColors.js";
+import { SCREEN_VIEW_MARGIN } from "../../SimConstants.js";
 import type { SimModel } from "../model/SimModel.js";
 import { SimScreenSummaryContent } from "./SimScreenSummaryContent.js";
 
-// Margin between screen edges and buttons/panels (in layout-bounds coordinates)
-const SCREEN_VIEW_MARGIN = 20;
-
 export class SimScreenView extends ScreenView {
   public constructor(model: SimModel, options?: ScreenViewOptions) {
     // ── Accessibility: screen summary ───────────────────────────────────────────