fix(line): clamp thickness <= length to avoid ^GB promotion

Zebra firmware promotes ^GB w,h,t with t > min(w,h) to max(w,t) x max(h,t),
so a vertical line emitted as ^GB t,length,t prints as a t x t square once
thickness exceeds length rather than the declared length x t band. Enforce
the t <= length invariant at every edit path:

- Thickness side-handle drag caps at p.length
- Property panel thickness gets max=p.length, length gets min=p.thickness
  (NumberInput already clamps typed values, so the cap is authoritative)
- Start/end handle drag commits cap thickness to the new length so
  shrinking a line below its current thickness pulls thickness down
  rather than violating the invariant

Existing stored lines with t > length keep their data and continue to
print the promoted square; touching any handle or input snaps them back
into a valid state. An explicit import-time warning for this case will
follow in a separate change.

Author: u8array

src/components/Canvas/LineObject.tsx

@@ -459,10 +459,18 @@ export function LineObject({
                 e.target.getParent(),
               );
               clearSnap();
+              // Shrinking the line below the current thickness would
+              // push the ZPL into the `^GB` promotion regime (t > length
+              // → printed `t × t`); cap thickness to the new length so
+              // the model preserves the t ≤ length invariant.
               onChange({
                 x: r.movingDotX,
                 y: r.movingDotY,
-                props: { length: r.length, angle: r.angle },
+                props: {
+                  length: r.length,
+                  angle: r.angle,
+                  thickness: Math.min(p.thickness, r.length),
+                },
               });
             }}
           />
@@ -520,7 +528,13 @@ export function LineObject({
                 e.target.getParent(),
               );
               clearSnap();
-              onChange({ props: { length: r.length, angle: r.angle } });
+              onChange({
+                props: {
+                  length: r.length,
+                  angle: r.angle,
+                  thickness: Math.min(p.thickness, r.length),
+                },
+              });
             }}
           />
           <Rect
@@ -549,9 +563,13 @@ export function LineObject({
               const extPx = isHorizontal
                 ? cursorY - lineCenterY
                 : cursorX - lineCenterX;
-              const newT = Math.max(
-                1,
-                Math.round(pxToDots(extPx, scale, dpmm)),
+              // Cap at p.length so the line never enters the ZPL ^GB
+              // promotion regime where thickness exceeds length and
+              // Labelary would print `t × t` rather than the band the
+              // user is dragging.
+              const newT = Math.min(
+                p.length,
+                Math.max(1, Math.round(pxToDots(extPx, scale, dpmm))),
               );
               setLiveThicknessDots(newT);
               // Pin the Rect to the (possibly-clamped) anchor so

src/registry/line.tsx

@@ -124,7 +124,10 @@ export const line: ObjectTypeDefinition<LineProps> = {
           <NumberInput
             label={t.registry.line.length}
             value={p.length}
-            min={1}
+            // Floored at thickness for the same reason thickness is
+            // capped at length below: keeps the model out of the ^GB
+            // promotion regime where t > length prints `t × t`.
+            min={p.thickness}
             onChange={(length) => onChange({ length })}
           />
           <NumberInput
@@ -140,6 +143,10 @@ export const line: ObjectTypeDefinition<LineProps> = {
           label={t.registry.line.thickness}
           value={p.thickness}
           min={1}
+          // Capped at length so the ZPL output stays out of the ^GB
+          // promotion regime (max(w, t) × max(h, t)), where the printer
+          // would extend the line beyond its declared length.
+          max={p.length}
           onChange={(thickness) => onChange({ thickness })}
         />