Add optional FixedScaleOffset dimension preservation

[shixi-li]

Closes #852.

Summary

• add an opt-in keep_dimensions parameter to FixedScaleOffset, with the existing flattened output retained by default
• preserve multidimensional shapes in both encode and decode, including logical coordinates when out uses the opposite C/F memory order
• include the option in codec config and repr, document it in the class docstring and release notes, and retain compatibility with legacy configs

Validation

• uv run pytest -q tests/test_fixedscaleoffset.py — 15 passed
• uv run pytest -q — 654 passed, 32 optional-dependency skips
• uv run ruff check src/numcodecs/fixedscaleoffset.py tests/test_fixedscaleoffset.py
• uv run ruff format --check src/numcodecs/fixedscaleoffset.py tests/test_fixedscaleoffset.py
• uv run mypy src/numcodecs/fixedscaleoffset.py tests/test_fixedscaleoffset.py
• git diff --check
• documentation build reached the updated API and release pages, but -W remains blocked by the current environment existing optional-codec imports and unrelated cross-reference warnings

Checklist

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for the modified user-facing class
• Changes documented in docs/release.md
• Docs build locally without existing warnings
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

AI assistance

Codex assisted with test planning and independent review. I ran the tests and quality checks, inspected the final diff, and addressed a cross-memory-order out regression found during review.