Add shared validation and versioned save recovery

[nolanvenhola]

What changed

• adds structured content diagnostics with stable codes, severity, owning content ID, resource path, property path, and message
• makes Zeliard Creator and the headless CI command use the same validation service, deterministic ordering, and formatter
• versions authored content definitions and introduces a separately versioned runtime/save-state model
• adds canonical JSON save envelopes with SHA-256 payload integrity
• adds deterministic in-memory v1-to-v2 migration with committed input and expected-output fixtures
• implements verified temporary writes, valid-primary backup rotation, corrupt-primary quarantine, and backup recovery
• documents save integrity, migration, atomic-write, and recovery guarantees

Acceptance evidence

• editor/CI diagnostic text is generated by the same service and remains identical regardless of content discovery order
• diagnostics link failures to owning resource, stable ID, and property
• the v1 fixture migrates repeatedly to the exact expected v2 semantic state and encoding
• tampered current saves fail checksum validation
• replacement saves retain the previous valid state as a backup
• corrupt primaries recover from backup without rewriting either source
• saving beside a corrupt primary quarantines it without replacing a recoverable backup
• corrupt saves with no backup fail without modifying the source file

Validation

• architecture and typing rules: PASS (34 scripts)
• production content graph: PASS (12 resources)
• validation/save scenarios: PASS (14 tests)
• Creator EditorPlugin startup: PASS
• main scene smoke: PASS
• Windows release export and executable smoke: PASS
• Web release export: PASS

Closes #241