fix(testdata): normalize diff artifacts to 813x1626 canvas

Several diff artifacts in testdata/diffs/ were stale or mismatched:
- gb_normal/gb_0_width/gb_0_height, postnl_qr, qr_code_offset: leftover
  400x640 diff images from an old canvas era; these labels are now
  PERFECT so the pipeline never regenerated them
- cf_font_designator, cf_font_no_orientation, code128_mode_d_fnc1,
  cp850_hex_chars, fo_lenient_coord, maxicode_default_mode2,
  maxicode_mode4: Labelary reference PNGs were 812x1624, so every
  diff run produced 1629x1626 comparisons and 812x1624 diffs

Fix: pad the 7 unit references to 813x1626 (same normalization the
bootstrap already applies to new fetches), drop the stale diff images,
and make the diff report self-healing — when a label has no diff,
remove any leftover diff image so old canvases can't linger.

Diff percentages drop for the 7 labels since the spurious 1px column
no longer counts (e.g. fo_lenient_coord 0.27% -> 0.02%, maxicode
labels 0.84%/0.83% -> 0.59%/0.58%). All e2e_golden tests still pass.

Author: GOODBOY008

testdata/diffs/bstc_diff.png

@@ -17,19 +17,19 @@
 ║ barcode128_mode_n_cba_sets     │ zpl  │    0.23% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
 ║ barcode128_mode_u              │ zpl  │    0.54% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
 ║ barcode128_rotated             │ zpl  │    0.24% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
-║ cf_font_designator             │ zpl  │    0.38% │      813x1626 │      812x1624 │ GOOD(<1%)    ║
-║ cf_font_no_orientation         │ zpl  │    0.43% │      813x1626 │      812x1624 │ GOOD(<1%)    ║
-║ code128_mode_d_fnc1            │ zpl  │    0.48% │      813x1626 │      812x1624 │ GOOD(<1%)    ║
-║ cp850_hex_chars                │ zpl  │    0.47% │      813x1626 │      812x1624 │ GOOD(<1%)    ║
+║ cf_font_designator             │ zpl  │    0.13% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
+║ cf_font_no_orientation         │ zpl  │    0.18% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
+║ code128_mode_d_fnc1            │ zpl  │    0.23% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
+║ cp850_hex_chars                │ zpl  │    0.23% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
 ║ dhlparceluk_dhl_text           │ zpl  │    0.23% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
 ║ dhlparceluk_ver                │ zpl  │    0.05% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
 ║ ean13                          │ zpl  │    2.76% │      813x1626 │      813x1626 │ MINOR(<5%)   ║
 ║ edi_triangle                   │ zpl  │    0.03% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
 ║ encodings_013                  │ zpl  │    1.56% │      813x1626 │      813x1626 │ MINOR(<5%)   ║
-║ fo_lenient_coord               │ zpl  │    0.27% │      813x1626 │      812x1624 │ GOOD(<1%)    ║
-║ font_p                         │ zpl  │    0.00% │      813x1626 │      813x1626 │ PERFECT      ║
-║ font_q                         │ zpl  │    0.00% │      813x1626 │      813x1626 │ PERFECT      ║
-║ font_s                         │ zpl  │    0.00% │      813x1626 │      813x1626 │ PERFECT      ║
+║ fo_lenient_coord               │ zpl  │    0.02% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
+║ font_p                         │ zpl  │    0.54% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
+║ font_q                         │ zpl  │    0.55% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
+║ font_s                         │ zpl  │    0.65% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
 ║ gb_0_height                    │ zpl  │    0.00% │      813x1626 │      813x1626 │ PERFECT      ║
 ║ gb_0_width                     │ zpl  │    0.00% │      813x1626 │      813x1626 │ PERFECT      ║
 ║ gb_normal                      │ zpl  │    0.00% │      813x1626 │      813x1626 │ PERFECT      ║
@@ -39,8 +39,8 @@
 ║ gd_thin_l                      │ zpl  │    0.03% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
 ║ gd_thin_r                      │ zpl  │    0.03% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
 ║ gs                             │ zpl  │    1.14% │      813x1626 │      813x1626 │ MINOR(<5%)   ║
-║ maxicode_default_mode2         │ zpl  │    0.84% │      813x1626 │      812x1624 │ GOOD(<1%)    ║
-║ maxicode_mode4                 │ zpl  │    0.83% │      813x1626 │      812x1624 │ GOOD(<1%)    ║
+║ maxicode_default_mode2         │ zpl  │    0.59% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
+║ maxicode_mode4                 │ zpl  │    0.58% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
 ║ mu_dpi_conversion              │ zpl  │    0.06% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
 ║ mu_millimeters                 │ zpl  │    3.18% │      813x1626 │      813x1626 │ MINOR(<5%)   ║
 ║ pdf417_basic                   │ zpl  │    0.29% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
@@ -69,5 +69,5 @@
 ║ usps_priority_mail             │ zpl  │    0.32% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
 ║ usps_test_merchant             │ zpl  │    0.07% │      813x1626 │      813x1626 │ GOOD(<1%)    ║
 ╠══════════════════════════════════════════════════════════════════════════════╣
-║ Summary: 65 total │ 8 perfect │ 48 good │ 7 minor │ 2 moderate │ 0 high │ 0 skip │ 0 err
+║ Summary: 65 total │ 5 perfect │ 51 good │ 7 minor │ 2 moderate │ 0 high │ 0 skip │ 0 err
 ╚══════════════════════════════════════════════════════════════════════════════╝