Refactor settings transaction boundary

[KeyffMS]

Summary

• extract SettingsNormalizer as the single migration, canonicalization, recovery, and reference-repair authority;
• reduce SettingsStore to JSON persistence and atomic file replacement while preserving the primary save exception during cleanup;
• publish defensive deep settings snapshots so UI/runtime code cannot mutate the committed in-memory state;
• introduce read-only settings contracts for resolver/query paths;
• replace broad ArgumentException/InvalidOperationException handling with explicit SettingsValidationException plus I/O failures;
• report and rethrow unexpected transaction exceptions;
• make failed generic commit values inaccessible and provide TryGetValue;
• add transaction-boundary regression tests and update architecture documentation;
• advance product metadata to 0.5.0.32.

Validation

Windows CI must pass the complete build, test suite, architecture checks, and self-contained publish validation.

Closes #35
Closes #36
Closes #39