Fix missing overhang layers with supported counterbore bridging

[mwz-iot]

Description

This PR fixes an issue where enabling Counterbore Hole Bridging - Partially Bridged could cause overhang layers that contact support to be missing.

Root cause:
PerimeterGenerator::process_no_bridge() treated the counterbore bridging candidate area as a special fill area and removed it from the original surface (last). Since last is later used to generate perimeter and overhang wall paths, removing this area caused the model body overhang layer to disappear when support was enabled.

Changes:

• When support is enabled, unsupported_filtered is no longer removed from last, so the original model surface is preserved and perimeter/overhang wall paths can still be generated.
• For the post-processing fill path, support-enabled cases now shrink the extra counterbore fill area inward before appending it to fill_surfaces.
• The shrink distance is based on the perimeter occupied zone:
  ext_perimeter_width / 2 + perimeter_spacing * (wall_loops - 1) + perimeter_spacing / 2
• This keeps counterbore bridging fill while preventing it from interfering with inner/outer walls.
• Support-disabled behavior remains unchanged.
• No UI or configuration semantics are changed.
• The surface type remains stInternal; this PR does not change it to stBottomBridge.

Screenshots/Recordings/Graphs

Before:

• With counterbore_hole_bridging = chbBridges and support enabled, the overhang layer touching support was missing.
• After preserving the surface, the fill could interfere with inner/outer wall paths.

After:

• The overhang layer is preserved.
• Counterbore fill is still generated.
• The fill area is inset from the perimeter zone and no longer interferes with inner/outer walls.

Tests

Verified with the reproduced 3MF model:

• counterbore_hole_bridging = chbNone

  • Layer 26 overhang layer exists as baseline.

• counterbore_hole_bridging = chbBridges with support enabled

  • Layer 26 overhang layer is preserved.
  • Counterbore fill remains present.
  • Fill no longer interferes with inner/outer walls.

• Support disabled with chbBridges

  • Original behavior is preserved.

Recommended regression checks:

• Compare chbNone and chbBridges on the same Z layer.
• Test different wall loop counts.
• Check chbFilled with support enabled if it shares the same post-processing path.