Add persistent filament-source and U1 toolhead routing model

[physicsG]

Summary

Introduce a first-class routing model that maps each logical project filament to a physical ACE source and one of the Snapmaker U1's four physical toolheads.

The U1 must remain a four-toolhead machine. ACE slots must not be represented as additional physical extruders.

Scope

• Add structured SourceId support.
• Add FilamentRoute and FilamentRoutingPlan domain types.
• Store project filament, source, physical toolhead, loaded/swap state, user pins, and inventory revision.
• Persist routing plans in 3MF projects.
• Extend existing mapping UI and serialization rather than introducing an unrelated workflow.
• Add compatibility validation for material, nozzle, temperature, remaining quantity, source state, and route reachability.
• Support user pins for filament-to-source, filament-to-head, and source-to-head relationships.
• Define stale-plan behavior after an inventory refresh.

Proposed code areas

src/libslic3r/FilamentRoutingPlan.hpp
src/libslic3r/FilamentRoutingPlan.cpp
src/libslic3r/ProjectTask.hpp
src/slic3r/GUI/AmsMappingPopup.hpp
src/slic3r/GUI/AmsMappingPopup.cpp
src/slic3r/GUI/SelectMachine.cpp
src/slic3r/GUI/filamentsync/*

Acceptance criteria

• Every logical project filament can be mapped to a structured source ID and physical head 0..3.
• No route can produce a physical head outside 0..3.
• Invalid material, nozzle, temperature, state, quantity, and reachability combinations are rejected with explicit reasons.
• User pins are preserved and conflicts are reported rather than ignored.
• Routing plans round-trip through 3MF without losing source IDs or pins.
• Inventory revision is stored with the routing plan.
• A changed or missing source marks the route stale.
• Same-head equivalent substitutions can be identified separately from changes that require reslicing.
• Unit tests cover serialization, validation branches, pins, and stale revisions.

Dependencies

• Depends on Add multiACE inventory provider and live source synchronization #1 for live inventory, but the routing model should also support manual/offline inventory fixtures.
• Required by native G-code generation, timing, and optimizer work.

Design references

• doc/multiace_u1_integration_design.md
• doc/multiace_testing_strategy.md