feat(keys_highway_3d): hit FX — vibrancy, timing-colored sparks, hit-line kick (K2) (#699)

- Note vibrancy: gem opacity 0.8 -> slider-driven (default 0.92),
  NOTE_EMISSIVE_BASE 0.08 -> 0.22, lane guides scale with the slider,
  _applyVibrancy retints the live scene without a chart rebuild
- Sparks (PORTED highway_3d, pool 96) at the struck key, colored by
  _timingHex/_classifyTiming (±100ms window, inner 40% = on-time; delta
  recovered from judgeHit's noteKey prefix — contract untouched);
  streak-scaled counts
- Hit-line brightness kick on scored presses (exp(-t*6) decay, hitFx
  slider), folded into the existing pulse incl. the bloom damp gate
- Settings: Hit sparks / Timing colours / Streak feedback + Hit feedback
  intensity + Note vibrancy sliders (keys3d_bg_*, live-applying)
- Tests: classifier boundaries, noteKey time round-trip, FX defaults
  (26 total)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

plugins/keys_highway_3d/settings.html

@@ -21,6 +21,59 @@
             sustains. Applies live; turn off to reclaim GPU headroom on
             weak machines.
         </p>
+
+        <label for="keysh3d-fx-sparks" class="flex items-center gap-2 text-xs text-gray-300 cursor-pointer mt-3">
+            <input type="checkbox" id="keysh3d-fx-sparks" checked
+                onchange="window.keys3dSetFx && window.keys3dSetFx('sparks', this.checked)">
+            Hit sparks
+        </label>
+        <p class="text-xs text-gray-500 mt-1">
+            A small particle burst on every scored key press, beside the
+            pitch-colored flame.
+        </p>
+
+        <label for="keysh3d-fx-timing" class="flex items-center gap-2 text-xs text-gray-300 cursor-pointer mt-3">
+            <input type="checkbox" id="keysh3d-fx-timing" checked
+                onchange="window.keys3dSetFx && window.keys3dSetFx('timingFx', this.checked)">
+            Timing colours
+        </label>
+        <p class="text-xs text-gray-500 mt-1">
+            Tint the sparks by timing — on-time green, early cyan, late
+            amber. Off = always green.
+        </p>
+
+        <label for="keysh3d-fx-streak" class="flex items-center gap-2 text-xs text-gray-300 cursor-pointer mt-3">
+            <input type="checkbox" id="keysh3d-fx-streak" checked
+                onchange="window.keys3dSetFx && window.keys3dSetFx('streakFx', this.checked)">
+            Streak feedback
+        </label>
+        <p class="text-xs text-gray-500 mt-1">
+            Spark bursts grow with your combo.
+        </p>
+
+        <label for="keysh3d-fx-hitfx" class="text-xs font-medium text-gray-400 mb-1 mt-3 block">
+            Hit feedback intensity <span id="keysh3d-fx-hitfx-val" class="text-gray-500 font-mono">0.70</span>
+        </label>
+        <input type="range" id="keysh3d-fx-hitfx"
+            min="0" max="1" step="0.05" value="0.7"
+            oninput="window.keys3dSetFx && window.keys3dSetFx('hitFx', this.value); document.getElementById('keysh3d-fx-hitfx-val').textContent = parseFloat(this.value).toFixed(2)"
+            class="w-full">
+        <p class="text-xs text-gray-500 mt-1">
+            Drives the hit-line brightness kick on scored presses. 0 turns
+            it off.
+        </p>
+
+        <label for="keysh3d-fx-vibrancy" class="text-xs font-medium text-gray-400 mb-1 mt-3 block">
+            Note vibrancy <span id="keysh3d-fx-vibrancy-val" class="text-gray-500 font-mono">0.85</span>
+        </label>
+        <input type="range" id="keysh3d-fx-vibrancy"
+            min="0" max="1" step="0.05" value="0.85"
+            oninput="window.keys3dSetFx && window.keys3dSetFx('vibrancy', this.value); document.getElementById('keysh3d-fx-vibrancy-val').textContent = parseFloat(this.value).toFixed(2)"
+            class="w-full">
+        <p class="text-xs text-gray-500 mt-1">
+            Note-gem solidity and lane-guide strength — lower to see more of
+            the keyboard through the notes.
+        </p>
     </div>
 
     <script>
@@ -33,12 +86,24 @@
             // FX toggles (keys3d_bg_* — guitar-parity graphics controls).
             // Only explicit values override; absent/corrupt keys keep the
             // default (ON), matching screen.js readFxSettings.
-            const storedBloom = localStorage.getItem('keys3d_bg_bloom');
-            if (storedBloom === '1' || storedBloom === 'true') {
-                document.getElementById('keysh3d-fx-bloom').checked = true;
-            } else if (storedBloom === '0' || storedBloom === 'false') {
-                document.getElementById('keysh3d-fx-bloom').checked = false;
-            }
+            const hydrateFxBool = (key, elId) => {
+                const raw = localStorage.getItem('keys3d_bg_' + key);
+                if (raw === '1' || raw === 'true') document.getElementById(elId).checked = true;
+                else if (raw === '0' || raw === 'false') document.getElementById(elId).checked = false;
+            };
+            hydrateFxBool('bloom', 'keysh3d-fx-bloom');
+            hydrateFxBool('sparks', 'keysh3d-fx-sparks');
+            hydrateFxBool('timingFx', 'keysh3d-fx-timing');
+            hydrateFxBool('streakFx', 'keysh3d-fx-streak');
+            const hydrateFxRange = (key, elId, valId) => {
+                const n = parseFloat(localStorage.getItem('keys3d_bg_' + key));
+                if (!Number.isFinite(n)) return;
+                const v = Math.min(1, Math.max(0, n));
+                document.getElementById(elId).value = String(v);
+                document.getElementById(valId).textContent = v.toFixed(2);
+            };
+            hydrateFxRange('hitFx', 'keysh3d-fx-hitfx', 'keysh3d-fx-hitfx-val');
+            hydrateFxRange('vibrancy', 'keysh3d-fx-vibrancy', 'keysh3d-fx-vibrancy-val');
         } catch (e) {
             console.warn('[Keys-Hwy3D settings] hydration failed:', e);
         }