feat: align project structure with PROJECT_STANDARDS.md

[mauryasameer]

• Restructure core/ → src/ with provider abstraction pattern (src/core/interfaces.py, src/providers/, src/services/, src/utils/)
• Extract MLX and PyTorch saliency backends into SaliencyProvider classes
• Add pyproject.toml with ruff config (replaces pytest.ini)
• Add VERSION (0.1.0), CHANGELOG.md, CLAUDE.md, task.md
• Reorganise tests into unit/, integration/, test_data/ subdirectories
• Update CI: Python 3.11+3.12 matrix + ruff lint job
• Pin ruff==0.11.2 in requirements-dev.txt
• Fix all ruff violations; add from __future__ import annotations for py3.9 compat
• Update README with version badge and new src/ structure section

[mauryasameer]

Summary

• Restructure core/ → src/ with provider abstraction pattern
• Extract MLX/PyTorch backends into SaliencyProvider interface classes
• Add pyproject.toml (replaces pytest.ini), VERSION, CHANGELOG.md, CLAUDE.md
• Reorganise tests into unit/, integration/, test_data/
• CI: Python 3.11+3.12 matrix + ruff lint job
• Fix all ruff violations, add from __future__ import annotations
• README: version badge + updated structure section

Test plan

• CI test job passes on 3.11 and 3.12
• CI lint job passes (ruff check . → zero issues)
• pytest tests/unit/ — 90 tests pass
• pytest tests/integration/ — structure tests pass