feat(m4): fixed tape line for length calibration (#99)

The photo pans, zooms, and tilts under a line that stays in the middle
of the frame. Scale is the source-pixel distance between the ticks
divided by the declared length, not the mask span.

frontend/src/capture/MeasurePanel.svelte

@@ -3,7 +3,6 @@
   import type { Capture } from './capture.types';
   import { inToEighths, eighthsToIn } from './capture.types';
   import { fmtEighths } from './session';
-  import { calibrationScale } from '../png/crop';
   import PhotoStage from './PhotoStage.svelte';
   import {
     buildPhotosReadyLineup,
@@ -55,12 +54,6 @@
     return Number.isFinite(inches) && inches > 0 ? inches : null;
   };
 
-  $: tapeScale = calibrationScale(
-    state.axis.tapeLine.start,
-    state.axis.tapeLine.end,
-    positiveInches(String(lengthIn)) ?? 0,
-  );
-
   const commitLength = () => {
     const inches = positiveInches(String(lengthIn));
     const eighths = inches === null ? 0 : inToEighths(inches);
@@ -74,7 +67,7 @@
     thicknessIn = eighths === 0 ? '' : String(eighthsToIn(eighths));
   };
 
-  $: canCalc = positiveInches(String(lengthIn)) !== null && positiveInches(String(thicknessIn)) !== null && tapeScale > 0;
+  $: canCalc = positiveInches(String(lengthIn)) !== null && positiveInches(String(thicknessIn)) !== null;
 
   const doCalc = async () => {
     calcBusy = true;
@@ -100,7 +93,7 @@
 
     <div class="fields">
       <div class="field">
-        <span class="label">Tape length L (distance between ticks)</span>
+        <span class="label">Length L (tape reading)</span>
         <input
           type="number"
           aria-label="Length L"
@@ -128,7 +121,6 @@
         />
       </div>
     </div>
-    <p class="scale">Tape ticks: {Math.hypot(state.axis.tapeLine.end[0] - state.axis.tapeLine.start[0], state.axis.tapeLine.end[1] - state.axis.tapeLine.start[1]).toFixed(1)} source-photo px · Scale {tapeScale.toFixed(2)} px/in</p>
 
     <div class="row">
       <button type="button" class="ghost" on:click={back}>Back</button>
@@ -202,7 +194,6 @@
   .fields { display: flex; gap: 16px; margin-top: 16px; }
   .field { flex: 1; }
   .label { display: block; font-size: 0.9rem; color: #aaa; margin-bottom: 8px; }
-  .scale { margin: 8px 0 0; color: #aaa; font-size: 0.85rem; font-variant-numeric: tabular-nums; }
   .field input {
     box-sizing: border-box; width: 100%; min-height: 52px; padding: 12px; font-size: 1rem;
     border-radius: 10px; border: 1px solid #444; background: #1d1d1d; color: #eee;