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/png/crop.ts

@@ -0,0 +1,21 @@
+/**
+ * Shared crop constants for inventory PNG + tape-line overlay (#99).
+ * Aspect is fixed 3:4; crop_fill only moves the long-axis end insets.
+ * Non-retro: changing fill affects future encodes only, not stored PNGs.
+ */
+
+export const DEFAULT_CROP_FILL = 0.9;
+export const ASPECT_W = 3;
+export const ASPECT_H = 4;
+export const FLOOR_PX = 800;
+
+/** Fraction of the long axis inset from each end: (1 - fill) / 2. */
+export function cropMarginFraction(fill: number = DEFAULT_CROP_FILL): number {
+  const f = clampFill(fill);
+  return (1 - f) / 2;
+}
+
+export function clampFill(fill: number): number {
+  if (!Number.isFinite(fill)) return DEFAULT_CROP_FILL;
+  return Math.min(1, Math.max(0.01, fill));
+}