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+# Implementation progress — PLAN.md
+
+The user asks for educational 3D graphics for eight subjects. PLAN.md is the
+subject specification; no separate design or implementation spec exists.
+
+Ruling: Implement the eight subjects as selectable experiments in the existing
+Three.js app, sharing a scene host and parameter/readout interface. This gives
+each topic an interactive visual immediately. The models are idealizations,
+explained in the UI, rather than general field/circuit/quantum solvers.
+
+Ruling: Continue in this directory because `.git` is an empty managed mount,
+not a usable Git repository; worktree/commit scripts cannot run here.
+
+Pre-flight: Potential and energy share Coulomb models; capacitance and
+permittivity share a parallel-plate model; current and semiconductors share
+carrier constants. All physics functions use SI units and rendering handles
+unit conversion and illustrative scales.
+
+- Baseline: existing 3 physics tests pass.
+- [x] Electrostatics: charge/force/field, flux, energy, potential/gradient.
+- [x] Matter/circuits: capacitance, dielectrics, current, semiconductors.
+- [x] Eight experiment definitions and educational UI.
+- [x] Eight 3D scenes, interaction, animation, cleanup.
+- [x] Tests, browser checks, build, README.
+
+Implementation:
+- Electrostatics complete: force/field, exact enclosed-charge flux (surface
+  singularity disclosed), interaction energy/work, potential and probe gradient.
+- Matter/circuits complete: parallel plates, fixed-V/fixed-Q dielectrics,
+  Ohmic wire with signed drift, intrinsic/n/p silicon equilibrium/bands.
+- UI and eight scenes complete: independent settings, deep links/history,
+  reset, pause/reduced motion, responsive layout and WebGL fallback.
+- Renderer rebuilds are coalesced; obsolete GPU resources are disposed.
+- Visual inspection: field, potential, capacitance and semiconductor screenshots
+  checked. Labels changed to stable screen sizes for readability.
+
+Final review: fresh read-only reviewer approved the current implementation;
+no Critical or Important findings.
+- Model direction-only capacitor-arrow disclosure was already added during
+  implementation and resolves the reviewer's corresponding suggestion.
+- Final: minor (deferred): browser pause test verifies the state/button;
+  pixel comparisons proving actual animation cessation are not yet included.
+- Final: minor (deferred): finite-difference potential-gradient unit test covers
+  one charge along x; add multiple sources/off-axis components when expanding.
+- Build has a non-fatal Three.js bundle-size advisory (~598 kB minified,
+  ~153 kB gzip). No deployment or code-splitting requirement in PLAN.md.
+
+Final verification: 20/20 unit tests, 10/10 Chromium browser tests, strict
+TypeScript and Vite production build pass after the final label adjustment.
+All eight scenes were checked for rendered geometry and browser exceptions.
+The app remains served on http://localhost:5173. No Git integration action
+was available or required in this non-repository workspace.