feat(vpto): add A2/A3 MGather lowering

[castigli]

Summary

• lower A2/A3 GM-to-UB pto.mgather for Row and Elem coalescing
• support Undefined, Zero, Clamp, and Wrap OOB modes with PTO-ISA uint32 index semantics
• validate payload types, tile layouts, static partition bounds, addressing overflow, and 32-byte row alignment
• expose pto.tile.mgather and add focused lit, PTODSL, and A3 numerical coverage

Validation

• LLVM 19.1.7 Docker build: cmake --build build --target PTOASPythonPackage -j4
• focused A2/A3 MGather lit RUN lines with FileCheck
• PTODSL wrapper unit test: 1 passed
• queued A3 Docker numerical tests: 8 passed (Row/Elem x Undefined/Zero/Clamp/Wrap)
• changed-code compliance: tracked changes clean; unrelated untracked Dockerfile excluded

[castigli]

Superseded by two independently reviewable PRs:

• Backend-only A2/A3 VPTO lowering: feat(vpto): add A2/A3 MGather lowering #1426
• Backend-neutral PTODSL API: feat(ptodsl): expose tile MGather #1427

Closing this combined draft to keep frontend and backend concerns separate.