Bound the parallel pressure-advance lookup to the filament count

Review findings on the preceding commit, plus one defect it should have
caught.

- The IDEX/IQEX pressure-advance loop fed resolve_filament_for_head()'s
  result straight into enable_pressure_advance and pressure_advance. That
  result is bounded by physical_extruder_map, which holds one entry per
  NOZZLE, while both options are indexed per filament SLOT. On a printer
  with more nozzles than the project has filaments the two spaces diverge
  and get_at() clamped the overflow onto filament 0, emitting its pressure
  advance on a secondary carriage. The second-layer temperature loop bounds the
  same lookup, but against nozzle_temperature, which is variant-expanded and so
  is not the slot count either -- it is not the precedent it looks like.
  IMEXHelpers.hpp states the rule
  once, and a test pins the contract that makes the bound necessary:
  resolve_filament_for_head() answers in nozzle space, so a non-negative
  result is not by itself safe to use as a filament id.

- The header claimed every caller renders a -1 tool qualifier as "emit
  none". RepRapFirmware substitutes the historical D0 instead, deliberately
  and with its own comment in GCodeWriter. Say so, rather than leaving a
  contract a future author would code against.

- A cross-reference pointed at a hard-coded line number that the preceding
  commit had itself shifted by nine lines. Name the function instead.

- The multi-color rejection reasons reach the user through Print::validate()
  as raw English, while the returns on either side of them use L(). Wrap
  them and register IMEXHelpers.cpp for extraction. They also still said
  "IMEX", the internal name, so they move to IDEX/IQEX with the rest of the
  user-facing strings rather than shipping the internal one to translators.

- Trim the preceding commit's comments. One block explained the same
  clamping hazard six times; the canonical explanation now lives in
  IMEXHelpers.hpp and the call sites point at it. The mode grid carried
  twelve lines of commentary and no code, most of it archaeology already in
  the commit message, and one claim about the modes editor that was not
  true. The ArrangeJob threading note stays: it documents an invariant that
  cannot be recovered from the code.

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

src/libslic3r/IMEXHelpers.cpp

@@ -11,9 +11,13 @@
 #include <boost/log/trivial.hpp>
 
 #include "libslic3r/ClipperUtils.hpp"
+#include "libslic3r/I18N.hpp"
 #include "libslic3r/PresetBundle.hpp"
 #include "libslic3r/PrintConfig.hpp"
 
+// Mark string for localization and translate.
+#define L(s) Slic3r::I18N::translate(s)
+
 namespace Slic3r {
 
 namespace {
@@ -60,14 +64,9 @@ int imex_pem_tool_for(int filament_id, const std::string& parallel_mode, const C
     const bool imex_parallel = !parallel_mode.empty() && parallel_mode != kImexPrimaryMode;
     if (!imex_parallel || pem.values.empty())
         return -1;
-    // Bounds-check rather than get_at(), for the same reason imex_physical_heater_for() below
-    // does: get_at() CLAMPS to values.front(), so a filament id past the end of the map would
-    // silently address physical head pem[0] instead of reporting "no mapping". The map is one
-    // entry per NOZZLE (effective_physical_extruder_map), while filament ids index filament
-    // SLOTS, and nothing caps the slot count at the nozzle count -- set_num_filaments() takes
-    // its size from the project, so opening a project authored with more filaments than this
-    // printer has extruders leaves ids past the end. Emitting no tool qualifier is correct
-    // there; pinning PA onto the primary's carriage is not.
+    // Bounds-checked, not get_at() -- see the note in IMEXHelpers.hpp for why the two index
+    // spaces diverge. Emitting no tool qualifier is correct for a miss; pinning PA onto the
+    // primary's carriage is not.
     if (filament_id < 0 || filament_id >= (int) pem.values.size())
         return -1;
     return pem.values[filament_id];
@@ -109,11 +108,11 @@ std::string imex_multicolor_block_reason(const std::string& parallel_mode,
             if (filament < 0 || filament >= (int)pem.values.size()) continue;
             const int phys = pem.values[filament];  // bounds-checked above; get_at would clamp
             if (!seen_physicals.insert(phys).second) {
-                return "Multi-color prints in IMEX parallel modes require each filament "
-                       "to have its own dedicated physical extruder. Two or more of the active "
-                       "filaments are routed to the same physical head via the printer's "
-                       "physical extruder map (an MMU/AFC manifold), which the slaved gantry "
-                       "cannot follow.";
+                return L("Multi-color prints in IDEX/IQEX parallel modes require each filament "
+                         "to have its own dedicated physical extruder. Two or more of the active "
+                         "filaments are routed to the same physical head via the printer's "
+                         "physical extruder map (an MMU/AFC manifold), which the slaved gantry "
+                         "cannot follow.");
             }
         }
     }
@@ -121,9 +120,9 @@ std::string imex_multicolor_block_reason(const std::string& parallel_mode,
     // Determine which physical head is the primary in this mode.
     const int primary_physical = imex_primary_tool_for_mode(active_tools_str);
     if (primary_physical < 0) {
-        return "The active IMEX mode does not define a primary tool, so multi-color "
-               "printing cannot be scheduled. Open the printer settings IMEX Modes "
-               "editor and assign a Primary role to one tool.";
+        return L("The active IDEX/IQEX mode does not define a primary tool, so multi-color "
+                 "printing cannot be scheduled. Open the printer settings IDEX/IQEX Modes "
+                 "editor and assign a Primary role to one tool.");
     }
 
     // Walk the active tools once: collect the set of distinct gantries spanned by
@@ -149,11 +148,11 @@ std::string imex_multicolor_block_reason(const std::string& parallel_mode,
     // (use Primary mode for that), and the user's mode_gcode would still emit
     // parallel-print firmware setup that doesn't apply here.
     if (active_gantries.size() < 2) {
-        return "Multi-color in this mode isn't a parallel-print scenario — all of the "
-               "active tools sit on a single gantry, so there's no second gantry being "
-               "copied or mirrored to. Switch the plate to Primary mode for multi-color "
-               "printing on a single gantry, or define an IMEX mode that includes "
-               "tools on a second gantry.";
+        return L("Multi-color in this mode isn't a parallel-print scenario — all of the "
+                 "active tools sit on a single gantry, so there's no second gantry being "
+                 "copied or mirrored to. Switch the plate to Primary mode for multi-color "
+                 "printing on a single gantry, or define an IDEX/IQEX mode that includes "
+                 "tools on a second gantry.");
     }
 
     // Dual-gantry mode but no Span tool on the primary's gantry — the slicer would
@@ -162,11 +161,11 @@ std::string imex_multicolor_block_reason(const std::string& parallel_mode,
     // copies (each gantry tool prints its own object) which is incompatible with
     // mid-print multicolor in a parallel mode.
     if (!span_on_primary_gantry) {
-        return "Multi-color prints in IMEX parallel modes require a Span tool on the "
-               "primary's gantry — without one, the mode doesn't declare a within-gantry "
-               "multicolor partner. Either reduce the print to a single filament, switch to "
-               "Primary mode, or open the printer settings IMEX Modes editor and mark a "
-               "tool on the primary's gantry as Span.";
+        return L("Multi-color prints in IDEX/IQEX parallel modes require a Span tool on the "
+                 "primary's gantry — without one, the mode doesn't declare a within-gantry "
+                 "multicolor partner. Either reduce the print to a single filament, switch to "
+                 "Primary mode, or open the printer settings IDEX/IQEX Modes editor and mark a "
+                 "tool on the primary's gantry as Span.");
     }
     return {};
 }
@@ -507,8 +506,8 @@ int resolve_filament_for_head(const std::map<int,int>& plate_map,
     auto it = plate_map.find(physical);
     if (it != plate_map.end()) {
         const int zero_based = it->second - 1;
-        // pem has one entry per logical filament slot, so its size IS the slot count and
-        // an override outside it names a filament that does not exist. Bounding here rather
+        // pem is indexed by filament slot here, so an override outside it names a filament
+        // this printer cannot route. Bounding here rather
         // than at the parse site is deliberate: the parser is handed a raw string with no
         // notion of how many filaments the project has, while every consumer of this
         // function's result indexes a per-filament array. The picker only ever offers