refactor(tigrbl): move ValidityWindow mixin from fields to lifecycle

[cobycloud]

Motivation

• Group lifecycle-related mixins together by moving the ValidityWindow mixin out of fields.py and into lifecycle.py so semantics align with other timestamp/version mixins.

Description

• Moved the ValidityWindow @declarative_mixin (with valid_from/valid_to column specs using tzutcnow / tzutcnow_plus_day) from pkgs/standards/tigrbl/tigrbl/orm/mixins/fields.py to pkgs/standards/tigrbl/tigrbl/orm/mixins/lifecycle.py without changing the column behavior.
• Added tzutcnow_plus_day import to lifecycle.py and removed now-unused datetime- and TZ-related imports from fields.py.
• Updated pkgs/standards/tigrbl/tigrbl/orm/mixins/__init__.py to import ValidityWindow from .lifecycle and removed it from the .fields import list while preserving the public __all__ surface.
• Cleaned formatting and minor style fixes to the affected modules.

Testing

• Ran ruff format . in pkgs/standards/tigrbl, which succeeded and reformatted 1 file.
• Ran ruff check . --fix in pkgs/standards/tigrbl, which succeeded and auto-fixed 2 issues.
• Attempted uv run --directory standards --package tigrbl ruff format . from pkgs/, which failed due to the workspace/source configuration preventing dependency resolution in this environment.

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Codex Task