PascalCase / legacy schema files load as an empty schema reported as a successful migration; saving from the editor then wipes the file #278

What's wrong

SchemaSerializer only reads camelCase property names (Schema/SchemaSerializer.cs:16-26). It sets PropertyNamingPolicy = CamelCase, does not set PropertyNameCaseInsensitive, and does not reject unmapped members. So every key in a document written with Classes, Enums, Name, Members, ... is silently ignored.

Such a file also has no formatVersion, so Load treats it as version 0 and returns Status = Success with Message = "Migrated from format version 0 to 7." (SchemaSerializer.cs:~181-203). The resulting schema has 0 classes, 0 enums and 0 validation issues.

Reproduction

Every file under samples/legacy/** (26 files: the repository's own Carbon Monoxide and Dungeoneer schemas) produces this:

var r = SchemaSerializer.Load(File.ReadAllText("samples/legacy/dungeoneer/weapon.schema.json"));
// Success | Migrated from format version 0 to 7. | classes=0 enums=0 issues=0

A hand-written { "Classes": [ { "Name": "A", ... } ] } behaves the same way.

Consequences

• kschema validate old.schema.json prints 0 error(s), 0 warning(s) and exits 0.
• kschema generate reports "declares no code generators" rather than saying the file wasn't understood.
• In the editor (Schema.Editor/SchemaEditor.Files.cs, LoadFrom), the file opens as an empty document with no message. Any edit followed by Save overwrites the original with an empty schema, and the data is lost.
• docs/schema-format.md:708-709 says "No version field - treat it as version 0 and migrate. Files written before versioning are still readable and always will be." For these files that promise is broken.

Not supporting the legacy dialect is deliberate (samples/README.md; LegacySchemaReader lives in the tests). Loading it as an empty success is not.

Suggested fix

• Refuse a document whose root has properties the model doesn't recognise. Either set JsonUnmappedMemberHandling.Disallow on Schema, or pre-check the known root keys in ReadFormatVersion. Return SchemaLoadStatus.InvalidJson with a message that names the key, e.g. "unrecognised property 'Classes' — property names are camelCase".
• Or set PropertyNameCaseInsensitive = true, so that casing alone never drops data.
• Either way, add a test that loading samples/legacy/dungeoneer/weapon.schema.json through SchemaSerializer.Load does not succeed with an empty schema.