feat(vpto): add A2/A3 gather lowering

[castigli]

Summary

• Add A2/A3 VPTO lowering for pto.tgatherb through pto.ub.vgatherb to packed llvm.hivm.VGATHERB.b16/.b32 calls.
• Lower pto.tgather index form through vmuls + vgather, preserving count-mode mask lifetime and chunking wide rows.
• Model A2/A3 tgatherb as a 32-byte block gather with one compact i32 block address per output block, padded to eight entries per repeat.
• Add UB op verification/effects, synchronization modeling, design notes, lit coverage, and NPU e2e tests.
• Build on the generic PTODSL/A5 tgatherb support merged in Add TGATHERB #932 without duplicating its wrapper or namespace registration.

Notes

• tgatherb is a 32-byte block gather, not scalar element gather.
• Scalar arbitrary-element gather is handled by the tgather index form and vgather.
• A2/A3 mgather is supported by PTO-ISA/EmitC; GM-to-UB VPTO lowering is intentionally split into a stacked follow-up because it requires scalar index/OOB handling and MTE2 decomposition.

Validation

• Hardened Docker ptoas build: passed.
• Gather-filter lit tests: 40 passed, including all 11 PR-specific gather tests.
• 910B2 NPU e2e on devices 2 and 3: 15 passed.

Covered NPU shapes:

• tgatherb f32: 1x8, 1x64, 1x96, 2x64, 1x128, 4x64
• tgatherb f16: 1x16, 1x128, 1x192
• scalar tgather f32/f16: 1x64, 2x64, 2x128

[gemini-code-assist]

Code Review

This pull request implements tile-level gather lowering (pto.tgather and pto.tgatherb) for the A2/A3 vpto backend, introducing new flat UB-pointer gather ops (pto.ub.vgather and pto.ub.vgatherb) and their lowerings to LLVM intrinsics with packed config words. It also adds synchronization macro models, extensive end-to-end tests, and lit verification tests. The review feedback suggests several improvements: emitting a compilation error instead of silently skipping lowering when srcAddr cannot be resolved, using safer defensive checks for validCol, explicitly casting mixed signed/unsigned types to prevent compiler warnings, and using castOp->getOperand(0) to avoid potential C++ name hiding issues.

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