Address PR review feedback: code style and validation fixes

[Copilot]

Addresses unresolved review comments from PR #2 review thread.

Changes

• Trailing whitespace cleanup: planewave.py, exchange_legendre.py, pyproject.toml
• Input validation: Added sign_magneticfield validation in get_form_factors() public API
• Dead code removal: Removed unused Veff = None assignment in exchange_legendre.py
• Documentation: Fixed grammar in README.md ("wavefunction used to find" → "wavefunction is used to find")
• Test accuracy: Fixed misleading comment in test_validation.py (tolerance is 3e-3, not 1e-4)
• Auto-formatting: Import ordering, lru_cache(maxsize=None) → cache, removed unnecessary string quotes from type annotations

All 18 tests pass.

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Pull request overview

This pull request addresses code quality and consistency issues across the quantumhall_matrixelements package. The changes are primarily focused on code cleanup, modernization, and improved validation, with no functional logic changes to the core algorithms.

Key changes:

• Modernized caching by replacing lru_cache(maxsize=None) with functools.cache (Python 3.9+ feature)
• Removed unnecessary string quotes from type annotations (leveraging from __future__ import annotations)
• Added input validation for sign_magneticfield parameter and cleaned up unused variable assignments
• Fixed code style issues including trailing whitespace, import ordering, and comment accuracy

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/test_validation.py	Updated comment to accurately reflect the 3e-3 tolerance used in assertion; added blank lines for PEP 8 compliance
tests/test_exchange_legendre.py	Fixed import statement ordering to follow alphabetical sorting convention
src/quantumhall_matrixelements/planewave.py	Removed unnecessary quotes from type annotations; added input validation for sign_magneticfield; removed trailing whitespace
src/quantumhall_matrixelements/exchange_legendre.py	Replaced lru_cache with cache decorator; removed unused Veff assignment in Coulomb case; removed trailing whitespace; removed unnecessary quotes from type annotations
src/quantumhall_matrixelements/exchange_hankel.py	Replaced lru_cache with cache decorator; removed unnecessary quotes from type annotations
src/quantumhall_matrixelements/diagnostic.py	Removed unnecessary quotes from type annotations
src/quantumhall_matrixelements/__init__.py	Fixed import ordering (standard library, then local imports); removed unnecessary quotes from type annotations
pyproject.toml	Removed trailing whitespace from authors line
README.md	Fixed grammar: changed "wavefunction used" to "wavefunction is used"

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