Generate native multiACE source-change G-code for the U1

[physicsG]

Summary

Generate correct U1 + multiACE G-code directly from the frozen routing plan, without relying on a post-processing rewrite step.

Logical project filaments may exceed four, but final physical tool commands must remain limited to the U1's four heads: T0 through T3.

Scope

• Resolve source and physical-head routing before final tool ordering and wipe-tower generation.
• Emit native source-change macros such as ACE_SWAP_HEAD when the required source is not already loaded.
• Preserve normal T0–T3 physical tool selection.
• Generate correct park, unload/load, tool-select, purge, and resume ordering.
• Avoid unnecessary source swaps when the requested source is already loaded.
• Integrate source transitions with purge and wipe-tower accounting.
• Define failure/cancellation-safe macro boundaries.
• Remove the need for the existing external multiACE G-code postprocessor for supported workflows.

Example behavior

PARK_TOOLHEAD TOOL=2
ACE_SWAP_HEAD HEAD=2 ACE=1 SLOT=2
T2
PURGE_FOR_SOURCE_CHANGE FROM=3 TO=6

Exact printer mechanics remain implemented in Klipper/multiACE macros; the slicer determines the route, command sequence, temperatures, purge, and accounting.

Acceptance criteria

• Final generated G-code never emits a physical tool command greater than T3.
• Source changes use structured route data from Add persistent filament-source and U1 toolhead routing model #2.
• No ACE_SWAP_HEAD command is emitted when the correct source is already loaded.
• Park/swap/select/purge order is deterministic and documented.
• Wipe-tower and purge behavior remains valid after physical-head assignment.
• Cancellation and macro failure points are clearly bounded.
• Generated files no longer require an external rewrite step.
• Golden tests cover same-head swaps, head-plus-source changes, already-loaded sources, repeated returns, and invalid routes.
• Integration tests verify all physical tool commands remain T0–T3.

Dependencies

• Depends on Add persistent filament-source and U1 toolhead routing model #2.
• Uses inventory state from Add multiACE inventory provider and live source synchronization #1.
• Timing and statistics work should consume the generated source-change events.

Design references

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