Mizan IR: cut over to KDL, delete OpenAPI envelope

Replaces the transitional OpenAPI 3.0 + `x-mizan-*` extensions
substrate with the canonical Mizan IR as KDL, per docs/AFI_ARCHITECTURE.md:
"KDL is the contract; everything else (REST envelopes, OpenAPI
documents, framework idioms) is sediment around it."

End-to-end cutover. No transitional path left on main.

Forward direction:
  cores/mizan-python/src/mizan_core/ir.py
    build_ir() walks mizan_core.registry, introspects Pydantic
    models directly (no JSON-Schema indirection), and emits the
    Mizan IR document. The KDL grammar is locked in this file's
    module docstring.

Backends emit KDL:
  backends/mizan-fastapi/src/mizan_fastapi/ir.py
    `python -m mizan_fastapi.ir <module>` — CLI entry point.
  backends/mizan-django/.../management/commands/export_mizan_ir.py
    `manage.py export_mizan_ir` — Django mgmt command.

Codegen consumes KDL:
  protocol/mizan-codegen/Cargo.toml: + kdl = "6"
  protocol/mizan-codegen/src/ir.rs: NamedType { Struct/List/Enum/Alias }
    + TypeShape { Primitive/Ref/List/Optional/Enum/Union } sum types,
    replacing the JsonSchema sprawl. KDL parser walks the
    `kdl::KdlDocument` tree into typed Rust structs.
  protocol/mizan-codegen/src/fetch.rs: subprocess command switches
    to the new IR-export entry points.
  All emit modules (stage1 / react / python / rust / vue / svelte /
    channels) port their type-walkers from JsonSchema to the new
    sum types — case analysis collapses substantially.

Substrate-honesty wins beyond the moat closure:
  - `int | bool` multi-arm unions land as `TypeShape::Union` (was
    silently coerced to "string" before).
  - `<CamelName>Output = list[T]` returns emit as named alias
    types instead of struct-shaped wrappers, so consumer code
    `.map()` works directly on the type.
  - Pydantic field defaults flow through to `default` properties
    in KDL, then back to non-optional shape in every target.

Deleted:
  - backends/mizan-fastapi/src/mizan_fastapi/{cli,schema}.py
  - backends/mizan-django/.../export_mizan_schema.py
  - openapi-bearing half of mizan/export/__init__.py (edge
    manifest generator preserved — separate concern).
  - tests/afi/schema_normalizer.py
  - tests/fixtures/{afi_schema.json, channels_schema.json}
  - tests/fixtures/js_* baseline directories.

Verification:
  - 20 mizan-codegen unit tests green (IR deserialization,
    byte-equivalence parity across stage1/rust/python/react/vue/svelte
    against fresh KDL-driven baselines, channels structural).
  - tests/rust/run_wire_parity.py: 12/12 probes green driving
    the binary end-to-end through KDL.
  - Blazr studio-ui typechecks against the regenerated React client.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

protocol/mizan-codegen/tests/rust_parity.rs

@@ -14,7 +14,7 @@ use mizan_codegen::fetch::parse_ir_from_str;
 
 fn load_ir() -> mizan_codegen::ir::MizanIR {
     let path = PathBuf::from(env!("CARGO_MANIFEST_DIR"))
-        .join("tests/fixtures/afi_schema.json");
+        .join("tests/fixtures/afi_ir.kdl");
     parse_ir_from_str(&std::fs::read_to_string(&path).unwrap()).unwrap()
 }
 
@@ -35,7 +35,7 @@ fn fixture_config() -> Config {
 
 fn read_baseline(rel: &str) -> String {
     let path = PathBuf::from(env!("CARGO_MANIFEST_DIR"))
-        .join("tests/fixtures/js_rust")
+        .join("tests/fixtures/baselines/rust")
         .join(rel);
     std::fs::read_to_string(&path)
         .unwrap_or_else(|e| panic!("read baseline {}: {e}", path.display()))