fix: Update color change lock stitch direction to match C# Loop C

Color change finish lock stitches now look FORWARD (Loop C) instead of
backward, aligning the knot with the stop event data for correct tension
when the machine halts for thread color changes.

Lock stitch direction breakdown:
- Loop A (Jump/Entry): Look forward - hides knot under upcoming stitches
- Loop B (End/Cut): Look backward - hides knot inside previous stitches
- Loop C (Color Change): Look forward - aligns with stop event data

Added detailed comments documenting which loop corresponds to each
lock stitch location in the code.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/workers/patternConverter.worker.ts

@@ -155,6 +155,12 @@ async function initializePyodide(pyodideIndexURL?: string, pystitchWheelURL?: st
 /**
  * Calculate lock stitch direction by accumulating movement vectors
  * Matches the C# logic that accumulates coordinates until reaching threshold
+ *
+ * Three use cases from C# ConvertEmb function:
+ * - Loop A (Jump/Entry): lookAhead=true - Hides knot under upcoming stitches
+ * - Loop B (End/Cut): lookAhead=false - Hides knot inside previous stitches
+ * - Loop C (Color Change): lookAhead=true - Aligns knot with stop event data
+ *
  * @param stitches Array of stitches to analyze
  * @param currentIndex Current stitch index
  * @param lookAhead If true, look forward; if false, look backward
@@ -481,6 +487,8 @@ for i, stitch in enumerate(pattern.stitches):
 
         if (jumpDist > FEED_LENGTH) {
           // Long jump - add finishing lock stitches at previous position
+          // Loop B: End/Cut Vector - Look BACKWARD at previous stitches
+          // This hides the knot inside the embroidery we just finished
           const finishDir = calculateLockDirection(stitches, i - 1, false);
           penStitches.push(...generateLockStitches(prevX, prevY, finishDir.dirX, finishDir.dirY));
 
@@ -498,6 +506,8 @@ for i, stitch in enumerate(pattern.stitches):
           );
 
           // Add starting lock stitches at new position
+          // Loop A: Jump/Entry Vector - Look FORWARD at upcoming stitches
+          // This hides the knot under the stitches we're about to make
           const startDir = calculateLockDirection(stitches, i, true);
           penStitches.push(...generateLockStitches(absX, absY, startDir.dirX, startDir.dirY));
 
@@ -561,7 +571,9 @@ for i, stitch in enumerate(pattern.stitches):
         console.log(`[PEN]   Next stitch: cmd=${nextStitchCmd}, isJump=${nextIsJump}, pos=(${nextStitchX}, ${nextStitchY})`);
 
         // Step 1: Add finishing lock stitches at end of current color
-        const finishDir = calculateLockDirection(stitches, i, false);
+        // Loop C: Color Change Vector - Look FORWARD at the stop event data
+        // This aligns the knot with the stop command's data block for correct tension
+        const finishDir = calculateLockDirection(stitches, i, true);
         penStitches.push(...generateLockStitches(absX, absY, finishDir.dirX, finishDir.dirY));
         console.log(`[PEN]   Added 8 finishing lock stitches at (${absX}, ${absY}) dir=(${finishDir.dirX.toFixed(2)}, ${finishDir.dirY.toFixed(2)})`);
 
@@ -626,7 +638,8 @@ for i, stitch in enumerate(pattern.stitches):
         console.log(`[PEN]   Added COLOR_END marker at (${jumpToX}, ${jumpToY})`);
 
         // Step 5: Add starting lock stitches at the new position
-        // Look ahead from the next stitch (which might be a JUMP we skipped, so use i+1)
+        // Loop A: Jump/Entry Vector - Look FORWARD at upcoming stitches in new color
+        // This hides the knot under the stitches we're about to make
         const nextStitchIdx = nextIsJump ? i + 2 : i + 1;
         const startDir = calculateLockDirection(stitches, nextStitchIdx < stitches.length ? nextStitchIdx : i, true);
         penStitches.push(...generateLockStitches(jumpToX, jumpToY, startDir.dirX, startDir.dirY));