feat(m4): crop_fill setting + tape-line prototype (#99)

Add global crop_fill percent (default 90, 1-100) as a settings key with
FastAPI 4xx validation. Wires the value through to the inventory PNG
encode (non-retro: stored PNGs untouched). Adds a throwaway
TapePrototype.svelte behind ?tape-prototype for the #99 design sandbox.

Gates: check_requirements PASS, check_coverage PASS, settings tests 8/8.

frontend/src/capture/session.ts

@@ -10,6 +10,7 @@ import { measure } from '../measure/measure';
 import { orientedRect } from '../geometry/rect';
 import { encodeInventoryPng, type InventoryInput } from '../png/inventory';
 import { encodeOverlayPng, type EncodeOverlayInput } from '../png/overlay';
+import { FILL_DEFAULT, fillFromSetting } from '../png/crop';
 import { decodeOrientedBitmap } from '../decodeOrientedBitmap';
 
 const DEBOUNCE_MS = 2000;
@@ -41,6 +42,8 @@ export interface CaptureSession {
   setSheet(s: Sheet): void;
   approveMask(): void;
   setAxisAngle(deg: number): void;
+  /** #99: global crop fill from settings (percent or fraction); drives the PNG crop. */
+  setCropFill(pctOrFraction: number): void;
   approveAxis(): void;
   setLength(e: Eighths): void;
   setThickness(e: Eighths): void;
@@ -83,6 +86,8 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
   let busyTimer: ReturnType<typeof setTimeout> | null = null;
   let pngUrl: string | null = null;
   let resumed = false;
+  // #99: global crop fill (fraction). Set from settings; FILL_DEFAULT (90%) until told otherwise.
+  let cropFill = FILL_DEFAULT;
 
   const set = (next: Capture) => {
     state = next;
@@ -487,6 +492,11 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
       void persist();
     },
 
+    setCropFill(pctOrFraction: number) {
+      // #99: non-retro — only future encodes use this; stored PNGs are untouched.
+      cropFill = fillFromSetting(pctOrFraction);
+    },
+
     approveAxis() {
       if (state.k !== 'axis') return;
       set({
@@ -523,7 +533,7 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
       let pngBlob: Blob;
       let belowFloor = false;
       try {
-        const r = await encodeInventory({ bitmap, mask: approvedMask, source });
+        const r = await encodeInventory({ bitmap, mask: approvedMask, source, fill: cropFill });
         pngBlob = r.blob;
         belowFloor = r.sourceBelowFloor;
       } catch {
@@ -593,7 +603,7 @@ export function createCaptureSession(opts: SessionOpts = {}): CaptureSession {
           const siblingBitmap = await decodeOrientedBitmap(photo.blob);
           let encoded: { blob: Blob; sourceBelowFloor: boolean };
           try {
-            encoded = await encodeInventory({ bitmap: siblingBitmap, mask: derived.mask, source: siblingSource });
+            encoded = await encodeInventory({ bitmap: siblingBitmap, mask: derived.mask, source: siblingSource, fill: cropFill });
           } finally {
             siblingBitmap.close();
           }