Start 2.1.1 development

[rossant]

Summary

• advance the repository development version from 2.1.0 to 2.1.1.dev0
• keep pyproject.toml, phy.__version__, and the editable package entry in uv.lock synchronized
• leave the README and documentation on 2.1.0, which remains the current stable release

Why

Development builds from master should no longer identify themselves as the immutable 2.1.0 release. The .dev0 version also ensures the production publish helper refuses an accidental upload.

Validation

• version consistency check across all three files
• uv lock --check
• uv run --frozen phy --version
• uv run --frozen pytest -q phy/utils/tests/test_plugin.py
• uv build and package metadata inspection
• git diff --check