Embedded schema does not distinguish flags from enums

Problem

The schema string that Yardl embeds in every binary/NDJSON file does not record whether an enum-like type is a !flags type. As a result, a !flags definition and an !enum definition are byte-for-byte identical in the embedded schema. Any consumer that works purely from the embedded schema (e.g. a dynamic/schema-only reader, or a third-party tool) cannot reproduce Yardl's own flags semantics, and cannot even reproduce Yardl's own NDJSON output for affected protocols.

Concrete differences between Yardl's SDK output and a schema-only reader on the models/test/unittests.yml protocols:

These are the only fidelity differences observed across all 29 protocols in unittests.yml; everything else round-trips byte-for-byte. (Discovered while prototyping a dynamic, schema-only binary reader.)

Proposed fix

Emit the flags marker in the protocol schema. Simplest is to give EnumDefinition a custom MarshalJSON (or add an isFlags field) so the embedded types[] carries it — mirroring the existing enum/flags distinction in TypeDefinitions.MarshalJSON.