Refactor file serialization: suffix-driven formats, public to_dict/from_dict

[raphaeltimbo]

Summary

Follow-up to #145. This refactors how ccp objects are saved to and loaded from files, replacing the per-class toml/json branching with a single shared mechanism.

Design

• The file extension decides the format. point.save("point.json") writes JSON, point.save("point.toml") writes TOML. The file_type parameter is removed, and an unsupported/missing extension raises a clear ValueError instead of silently truncating the target file (save) or leaving parameters unbound (load).
• One shared module — ccp/data_io/serializers.py holds the suffix→(load, dump) registry, save_dict/load_dict helpers, and a Serializable mixin. Adding a future format is one line in the registry.
• Public to_dict()/from_dict() on Point, Curve, Impeller, StraightThrough and BackToBack (replacing private _dict_to_save/_dict_from_load). This gives an in-memory serialization API — used by the app to embed impellers inside .ccp zips without temp files.
• Atomic writes — data is dumped to a temp file and os.replaced onto the target, so a failed save never corrupts an existing file.
• Versioned files — every saved file records ccp_version (same versioning scheme the app's convert() migration helper already uses), so future format changes can be migrated. Curve/Impeller files nest points under a points table; legacy flat files (and files without ccp_version) still load.

Bug fix

The app pages pass io.StringIO to Impeller.load / StraightThrough.load / BackToBack.load when restoring .ccp session files. For Impeller this has been broken since #145 (load now calls open() on its argument). Pages 1–4 and test_app now parse the toml in memory and use the public from_dict.

Breaking changes

• save(file_name, file_type=...) → save(file_name); the filename must have a .toml or .json extension (no silent extension appending).
• load no longer accepts file-like objects — use from_dict(toml.loads(...)) for in-memory data.
• _dict_to_save/_dict_from_load are replaced by public to_dict/from_dict.
• StraightThrough.save/BackToBack.save gain json support via the mixin; their file parameter is now file_name.

Tests

• Round-trip save/load parametrized over toml and json for Point, Curve (previously untested) and Impeller.
• Unsupported-format error case, and a legacy flat-file load test.
• Full suite: 159 passed (pytest ccp/tests -n 4 --dist loadfile), including the app integration tests against the real example .ccp files.

Notes for review

• Part of the app-page diff is whitespace-only: ruff format applied to files that were not ruff-clean on main (hide-whitespace view recommended).
• The 9 doctest failures in the fast tier (-m "not slow") pre-date this branch and are unrelated (verified on clean main); worth a separate fix.