feat(capture): shared crop_fill + tape geometry for #99

- One crop_fill (default 0.9) replaces duplicated FILL in inventory/overlay
- Thin localStorage settings store; crop % only moves long-axis end insets
- Pure tapeGeometry: fixed stage line, stage↔image map, tick span → px
- measure() prefers tape tick span for scale; maskSpanAlong demoted
- rotateCalibration bakes cal tilt into inventory + overlay only
- Fix stale 80%-fill comment on slabBounds

frontend/src/settings/cropFill.ts

@@ -0,0 +1,55 @@
+/**
+ * Thin frontend-only crop_fill setting (#99).
+ * Default 0.9 — replaces duplicated FILL = 0.9 in png/inventory + png/overlay.
+ * Not a full M3 Settings surface; localStorage so it survives reloads.
+ */
+import { DEFAULT_CROP_FILL, clampFill } from '../png/crop';
+
+const KEY = 'slabuploader.crop_fill';
+
+let cached: number | null = null;
+
+export function getCropFill(): number {
+  if (cached != null) return cached;
+  if (typeof localStorage === 'undefined') {
+    cached = DEFAULT_CROP_FILL;
+    return cached;
+  }
+  try {
+    const raw = localStorage.getItem(KEY);
+    if (raw == null) {
+      cached = DEFAULT_CROP_FILL;
+      return cached;
+    }
+    cached = clampFill(Number.parseFloat(raw));
+    return cached;
+  } catch {
+    cached = DEFAULT_CROP_FILL;
+    return cached;
+  }
+}
+
+export function setCropFill(fill: number): number {
+  const next = clampFill(fill);
+  cached = next;
+  if (typeof localStorage !== 'undefined') {
+    try {
+      localStorage.setItem(KEY, String(next));
+    } catch {
+      /* private mode / quota — in-memory still updated */
+    }
+  }
+  return next;
+}
+
+/** Test helper: clear cached value (and optional storage). */
+export function _resetCropFillForTests(): void {
+  cached = null;
+  if (typeof localStorage !== 'undefined') {
+    try {
+      localStorage.removeItem(KEY);
+    } catch {
+      /* ignore */
+    }
+  }
+}