Stop estimating a prime tower for a single used filament (#15760)

# Description

A plate using one filament no longer reserves or draws a prime tower in
the plater just because multiple filament slots are configured. The
shared footprint estimate gated "no tower" on the purge volume being
zero, but on non-Bambu printers with the shipped defaults that volume
comes from the SEMM flush matrix, which reads every configured slot and
stays nonzero even when only one filament is used — so the preview
showed a tower the print could never contain. The estimate now decides
from the filament count alone, with wrapping detection and smooth
timelapse staying the only reasons a single-filament plate keeps a
tower, matching `Print::has_wipe_tower()`. The gate also moved above the
flush-volume scan so the lone-filament path no longer pays for it.

No change to slicing output — toolpath generation already omitted the
tower for a single used filament; this removes the phantom preview,
placement reservation, and validation footprint.

# Screenshots/Recordings/Graphs
Before the fix:


https://github.com/user-attachments/assets/c5b0ea61-127d-408e-bafe-f1f76c58b996

After the fix:


https://github.com/user-attachments/assets/ac6632b3-5d44-4013-aca3-73f6cfd1139c



## Tests

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changes made in this PR.
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Added a shipped-defaults single-filament case to
`tests/libslic3r/test_wipe_tower_estimate.cpp`: it estimates a 5.04
mm-deep tower before the fix and zero depth after. `[WipeTowerEstimate]`
and the fff_print `[WipeTower]` suites pass.

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src/libslic3r/GCode/WipeTowerEstimate.cpp

@@ -107,6 +107,12 @@ WipeTowerFootprint estimate_wipe_tower_footprint(const ConfigBase &config, WipeT
     // normalize_fdm_2 clearing enable_prime_tower. Its mixed-filament case is not modelled.
     const bool   need_wipe_tower  = smooth_timelapse || wrapping;
 
+    // Fewer than two filaments cannot make a tool change, so only wrapping detection or smooth
+    // timelapse print a tower then. The flush volume is no proof of one: it is read from the
+    // matrix of every configured slot, nonzero even when a single one of them is used.
+    if (filaments_cnt < 2 && !need_wipe_tower)
+        return footprint;
+
     // A tower printed for one of the reasons above has no tool change to purge for; both
     // planners give it the idle depth below and nothing more.
     const size_t purge_count = filaments_cnt > 1 ? (dual_nozzle ? filaments_cnt : filaments_cnt - 1) : 0;
@@ -151,14 +157,6 @@ WipeTowerFootprint estimate_wipe_tower_footprint(const ConfigBase &config, WipeT
             purges[longest_ramming].filament_change_length = float(float_at("filament_change_length", filament_ids[longest_ramming], 0.) * double(nozzles.size() - 1));
     }
 
-    // Both wall types decide this together: over-reserving only wastes bed area, but reporting
-    // no tower for one that is built collapses the validation hull to a point.
-    // A tool change is a reason on its own (see the base commit); Type1 already reserves
-    // per filament, Type2 has only the volume, which can resolve to zero.
-    const bool has_purge = type1 ? !purges.empty() : volume > EPSILON;
-    if (!has_purge && filaments_cnt < 2 && !need_wipe_tower)
-        return footprint;
-
     const double min_depth      = WipeTower::get_limit_depth_by_height(float(max_object_height));
     const float  perimeter_width = float(nozzle_diameter) * 1.25f; // Width_To_Nozzle_Ratio
     // With nothing to purge, plan_tower_new sizes the tower for wrapping detection or the
@@ -171,7 +169,10 @@ WipeTowerFootprint estimate_wipe_tower_footprint(const ConfigBase &config, WipeT
         if (!purges.empty())
             side = WipeTower::estimate_rib_tower_bbox_side(purges, float(width), float(layer_height), float(nozzle_diameter), float(extra_spacing), float(rib_width), float(extra_rib_length), float(max_object_height));
         else {
-            const double square = has_purge ? std::sqrt(volume / layer_height * extra_spacing) : idle_depth;
+            // Type2 squares the tower from its purge volume; Type1 with no purge list (a lone
+            // filament kept for timelapse or wrapping) sizes for the idle depth.
+            const bool   has_purge = !type1 && volume > EPSILON;
+            const double square    = has_purge ? std::sqrt(volume / layer_height * extra_spacing) : idle_depth;
             side = WipeTower::rib_footprint_side(float(square), float(square), float(rib_width), float(extra_rib_length), float(max_object_height));
         }
         footprint.width = footprint.depth = side;

tests/libslic3r/test_wipe_tower_estimate.cpp

@@ -335,6 +335,10 @@ TEST_CASE("The shipped defaults size the tower from the flush matrix", "[WipeTow
     const double flush_volume = WipeTower2::estimate_semm_flush_volume(config, 2);
     const double expected     = std::max(double(WipeTower::get_limit_depth_by_height(5.f)), flush_volume / (0.2 * 50.));
     CHECK_THAT(estimate(config, 2, 0.2, 5.).depth, WithinAbs(expected, 1e-6));
+
+    // The flush volume is nonzero for one slot, but a lone filament makes no tool change.
+    REQUIRE(WipeTower2::estimate_semm_flush_volume(config, 1) > 0.);
+    CHECK_THAT(estimate(config, 1, 0.2, 5.).depth, WithinAbs(0., 1e-9));
 }
 
 TEST_CASE("A config missing a tower key falls back to that key's default", "[WipeTowerEstimate]") {