Add write strategy policy

[Mattie]

Summary

• Add write_strategy policy with in_place as the default and atomic as an opt-in strategy.
• Wire the option through stashes, models, Model.Meta, create_model, sync, and direct Snapshot construction.
• Document the Windows-friendly default and atomic tradeoff for snapshot metadata writes.

Why

Atomic temp-file replacement is better for file visibility and failure preservation, but it can fail on Windows when active apps or readers hold the destination file open. The default now matches datafiles-style in-place writes while preserving atomic replacement for callers that choose it.

Validation

• python.exe -m compileall src\snapclass tests
• python.exe -m pytest tests\test_sessions_and_frozen.py tests\test_conversion_and_writes.py tests\test_stash_binding_and_api.py tests\test_model_meta.py tests\test_sync.py tests\test_snapshot_locking.py tests\test_snapshot_magic_and_serializers.py
• python.exe -m pytest

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[Copilot]

Pull request overview

This PR introduces a write_strategy policy to control how snapshot files are written, defaulting to Windows-friendly in-place writes while allowing callers to opt into atomic temp-file replacement. It threads the new option through stash/model configuration, runtime helpers, and updates documentation and tests accordingly.

Changes:

• Add write_strategy propagation via Stash, snapclass/create_model/sync, Model.Meta, and Snapshot configuration.
• Implement strategy-aware writing (in_place vs atomic) and default to in_place.
• Expand test coverage for strategy scoping/precedence and document tradeoffs; bump version to 0.1.4.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/test_sync.py	Adds a regression test ensuring sync(..., write_strategy=...) is accepted and used.
tests/test_stash_binding_and_api.py	Extends stash option plumbing tests to cover write_strategy.
tests/test_snapshot_magic_and_serializers.py	Ensures atomic writes are used in a concurrent conflict test scenario.
tests/test_snapshot_locking.py	Updates locking/symlink test coverage for atomic writes.
tests/test_sessions_and_frozen.py	Adds tests for default in-place behavior, atomic opt-in, and policy precedence.
tests/test_model_meta.py	Adds tests for Model.Meta.snapshot_write_strategy precedence and create_model(..., write_strategy=...).
tests/test_conversion_and_writes.py	Updates atomic-replace failure test to explicitly opt into atomic strategy.
src/snapclass/stash.py	Adds stash-level write_strategy policy, normalization, and inheritance resolution.
src/snapclass/schemas.py	Wires write_strategy through public APIs/config and implements strategy-aware _write_text.
README.md	Documents the new default and how/when to opt into atomic replacement.
pyproject.toml	Bumps project version to 0.1.4.

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[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated no new comments.