Add --focus option for profile scope filtering (F-PROFILE-003)

[noahgift]

Summary

The apr run --profile command profiles all operations. Add --focus option to limit profiling scope to specific components.

Requested Feature

Add --focus <COMPONENT> option to filter profile output:

apr run model.gguf -p "Hello" --profile --focus attention
apr run model.gguf -p "Hello" --profile --focus mlp
apr run model.gguf -p "Hello" --profile --focus embedding

Use Case

• Targeted performance analysis
• Debugging specific layer types
• Reduced output noise when investigating specific bottlenecks

Falsification Gate

This blocks F-PROFILE-003 in apr-model-qa-playbook certification:

"Focus filtering works - --focus attention limits scope"

Acceptance Criteria

• --focus attention only shows attention-related profiling
• --focus mlp only shows FFN/MLP-related profiling
• Invalid focus values produce helpful error message

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Filed from apr-model-qa-playbook certification process.

[noahgift]

Fixed: Added filter_results_by_focus() function in profile.rs. Supports filtering by: attention, mlp, matmul, embedding.