Fix 4-bit split QKV projection

[abhinav-bellapu]

Fixes #737.

Summary

• Add a shared 4-bit Q/K/V projection helper for regular attention.
• Preserve the existing flattened 4-bit projection behavior for normal [batch, pos, d_model] inputs.
• Handle split Q/K/V inputs by selecting the matching input-head/output-head projection slice, matching complex_attn_linear.
• Add CPU-safe regression coverage with fake bitsandbytes matmul, so the split 4-bit path is tested without requiring a quantized 7B model locally.

Why

The old 4-bit path projected split inputs to [batch, pos, input_head, d_model] and then tried to reshape directly to [batch, pos, head, d_head], which reproduces the shape error in #737. Split Q/K/V inputs need head-specific projections: each input head should keep only the corresponding output-head slice.

Validation

• make format
• make check-format
• uv run --python 3.11 pytest tests/unit/components/test_attention.py -q
• uv run --python 3.11 mypy .
• make unit-test