Add interactive GP concrete strength explorer (BOxCrete website)

GitHub Pages site at docs/ providing an interactive web UI for exploring
concrete mix compressive-strength predictions from the Gaussian process
model. The page runs the GP entirely in the browser via a WASM build of
the BLAS routines and a small JS port (gp.mjs).

Highlights
- Composition sliders with SI/imperial unit toggle
- Performance-tradeoff scatter (GWP, cost, strength) with axis switches,
  filters, and Pareto-optimal labelling
- Predicted strength curve with \u00b12\u03c3 uncertainty band and far-from-data
  warning
- Per-mix and per-ingredient insight panels (mix_analyses.json)
- About modal, citation copy buttons (BibTeX/APA), references panel
- Light/dark themes with frosted-glass header and animated gradient
- Mobile-first responsive layout: unified scatter/sliders panel with
  tab-style toggle, viewport-fit panels, evenly-spaced header items,
  iOS background fixes via fixed bg-layer
- Header restructured into h1 + .header-links (centered nav) +
  .header-actions wrapper so right-anchored controls survive when
  individual children are hidden on mobile

Supporting infrastructure
- scripts/export_model.py: serializes the trained GP to JSON for the web UI
- docs/model/*.json: exported GP parameters, training data, mix analyses
- docs/blas_f64.{wasm,js}: WASM glue for matrix ops in the browser
- test/test_js_gp.mjs: parity test for the JS GP against the Python model
- .github/workflows/js-sync.yml: CI to keep JS GP in sync

Author: SebastianAment

.github/workflows/js-sync.yml

@@ -0,0 +1,40 @@
+# Verifies that the JavaScript GP implementation matches Python reference predictions.
+# Catches drift if model parameters or JS code are changed without re-syncing.
+
+name: JS Model Sync
+
+# Restrict GITHUB_TOKEN to the minimum needed: read-only access to repo
+# contents (required by actions/checkout). The workflow never writes to the
+# repo, comments on PRs, deploys, or interacts with any other GitHub APIs.
+permissions:
+  contents: read
+
+on:
+  push:
+    branches: [main, master]
+    paths:
+      - 'docs/gp.mjs'
+      - 'docs/model/**'
+      - 'test/test_js_gp.mjs'
+  pull_request:
+    branches: [main, master]
+    paths:
+      - 'docs/gp.mjs'
+      - 'docs/model/**'
+      - 'test/test_js_gp.mjs'
+
+env:
+  FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: true
+
+jobs:
+  js-model-sync:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+
+      - name: Run JS GP sync test
+        run: node test/test_js_gp.mjs