Share the prime-tower rule with the slicer instead of copying it

imex_wipe_tower_hull() open-coded "does this plate print a tower?" as one
filament and no forcing reason. normalize_fdm_2, which is what actually
clears enable_prime_tower before slicing, has a second arm the copy omitted,
and reads the mixed-filament flag project-wide where the copy read the
plate. Two cases went wrong in opposite directions.

A ByObject plate with several objects prints no tower and the scene draws
none, yet the copy validated one and could refuse the slice with "the prime
tower overlaps an area reserved for IDEX/IQEX parallel printing" - with
nothing on screen to move.

A plate using one plain slot while some other slot in the project is a blend
does print a tower, because normalize_fdm_2 keeps it for any mixed filament
in the project, and the copy skipped validation entirely, so the tower could
be placed in a carriage zone and sliced.

prime_tower_is_printed() in libslic3r states the rule once, and both the
gate and a test use it. The counts are the ones normalize_fdm_2 is handed:
filament slots as authored, so a mixed slot counts once, and distinct
objects rather than instances. filament_is_mixed is a project option, so it
is passed in rather than read from the print preset.

The test drives every combination the rule looks at and compares the verdict
against normalize_fdm_2 itself, so the two cannot drift again without
failing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/libslic3r/test_wipe_tower_estimate.cpp

@@ -6,6 +6,7 @@
 #include "libslic3r/GCode/WipeTower2.hpp"
 #include "libslic3r/GCode/WipeTowerEstimate.hpp"
 #include "libslic3r/PrintConfig.hpp"
+#include "libslic3r/FilamentMixer.hpp"
 
 #include <cmath>
 #include <numeric>
@@ -381,3 +382,41 @@ TEST_CASE("A config missing a tower key falls back to that key's default", "[Wip
                             print_config_def.get("wipe_tower_extra_spacing")->default_value->clone());
     CHECK_THAT(estimate(partial, 3, 0.2, 5.).depth, WithinAbs(estimate(defaulted, 3, 0.2, 5.).depth, 1e-9));
 }
+
+TEST_CASE("prime_tower_is_printed answers exactly what normalize_fdm_2 decides", "[WipeTowerEstimate]") {
+    // The estimate says how big a tower is; this says whether there is one, and the authority is
+    // normalize_fdm_2, which clears enable_prime_tower before the plate is sliced. Pre-slice
+    // consumers cannot call it (it mutates a config), so the rule is mirrored - and mirrored rules
+    // drift, which is what this pins. Every combination the rule looks at, both verdicts compared.
+    const int  used_filaments = GENERATE(1, 2, 3);
+    const bool has_mixed      = GENERATE(false, true);
+    const int  num_objects    = GENERATE(1, 2);
+    const bool by_object      = GENERATE(false, true);
+    const bool smooth         = GENERATE(false, true);
+    const bool wrapping       = GENERATE(false, true);
+
+    DynamicPrintConfig config = preset_shaped_defaults();
+    config.set_key_value("enable_prime_tower", new ConfigOptionBool(true));
+    config.set_deserialize_strict("print_sequence", by_object ? "by object" : "by layer");
+    config.set_deserialize_strict("timelapse_type", smooth ? "1" : "0");
+    config.set_key_value("enable_wrapping_detection", new ConfigOptionBool(wrapping));
+    config.set_key_value("filament_is_mixed", new ConfigOptionBools(
+        has_mixed ? std::vector<unsigned char>{0, 1} : std::vector<unsigned char>{0, 0}));
+    REQUIRE(has_any_mixed_filament(config.option<ConfigOptionBools>("filament_is_mixed")->values) == has_mixed);
+
+    DynamicPrintConfig normalized = config;
+    normalized.normalize_fdm_2(num_objects, used_filaments);
+    const bool slicer_prints_one = normalized.opt_bool("enable_prime_tower");
+
+    CHECK(prime_tower_is_printed(config, used_filaments, num_objects, has_mixed) == slicer_prints_one);
+}
+
+TEST_CASE("prime_tower_is_printed follows the option the user set", "[WipeTowerEstimate]") {
+    DynamicPrintConfig config = preset_shaped_defaults();
+    config.set_key_value("enable_prime_tower", new ConfigOptionBool(false));
+    CHECK_FALSE(prime_tower_is_printed(config, 2, 1, false));
+
+    // Below one filament normalize_fdm_2 leaves the option alone, so this does too.
+    config.set_key_value("enable_prime_tower", new ConfigOptionBool(true));
+    CHECK(prime_tower_is_printed(config, 0, 1, false));
+}