Handle duplicate entry point scripts

[Sean-Kenneth-Doherty]

Fixes #315.

Summary

• track script paths generated from entry points during installation
• if the wheel also contains a .data/scripts file for the same path, warn and skip that wheel-provided script instead of letting it overwrite the generated entry point script
• add regressions for both the core install flow and the real filesystem destination

Validation

• uv run --isolated --with pytest --with pytest-xdist --with coverage --with pytest-cov --with-editable . python -m pytest --cov=installer --cov-fail-under=100 --cov-report=term-missing --cov-context=test -n auto tests -q (152 passed, 100% coverage)
• uv run --isolated --with ruff==0.15.12 ruff check src/installer/_core.py tests/test_core.py
• uv run --isolated --with ruff==0.15.12 ruff format --check src/installer/_core.py tests/test_core.py
• uv run --isolated --with mypy==2.0.0 --with httpx --with-editable . mypy src/installer/_core.py
• uv run --isolated --with-editable . python -m compileall -q src/installer/_core.py tests/test_core.py
• git diff --check origin/main...HEAD && git diff --check

Hosted CI, Read the Docs, and pre-commit.ci are green on this PR.

Assisted-by: OpenAI Codex