Rebrand TypeDef to AlkType in code, keyword strings, and docs

Rename all 19 JSON Schema custom keyword strings from "TypeDef:*"
to "AlkType:*" (e.g., "TypeDef:Struct" -> "AlkType:Struct")
across source, tests, and docs. This is a breaking change to the
schema format itself — existing schemas using the old keywords
must be updated.

Rename the Rust identifiers:
- TypedefEngine -> AlkTypeEngine
- TypedefError -> AlkTypeError
- TypeDefKind -> AlkTypeKind
- TYPEDEF_PREFIX -> ALKTYPE_PREFIX
- get_typedef_kind{,_loose,_loose_enum,_enum} ->
  get_alktype_kind{,_loose,_loose_enum,_enum}

Update error message strings ("unknown TypeDef kind" ->
"unknown AlkType kind"), 11 test function names containing
typedef_kind/to_typedef_error, and doc-comment prose ("TypeDef
kind" -> "AlkType kind", "typedef engine" -> "alktype
engine", "typedef schema" -> "alktype schema"). Fix the broken
docs/architecture/crates/typedef/ path references in source doc
comments to point at docs/architecture/ directly. Rebrand the
typedef:annotation test fixture and the "not-a-typedef" test
string to their alktype equivalents.

Update ~20 generic "typedef" prose references in the architecture
docs ("typedef is the binary struct engine", "use typedef",
"typedef limitation", "replaced by typedef", etc.) to alktype.
Rename TypedefEngine in the ADR-007 code example to AlkTypeEngine.

Preserve as provenance per the prior prose-rebrand decision:
typedef.ts references (external TypeBox source file),
docs/research/alknet-typedef/findings.md research citations,
/workspace/alknet-typedef-poc/ POC path, and the
"alknet-typedef:" research section headers in findings.

Build, 295 tests, and clippy all pass clean.

docs/architecture/questions/003-builder-api-for-schema-construction.md

@@ -9,7 +9,7 @@
   changing the existing JSON-consumption path)
 - **Priority**: medium
 - **Impacts**: Blocks programmatic schema construction in Rust without a
-  JS toolchain. Any consumer that wants to build typedef schemas at
+  JS toolchain. Any consumer that wants to build alktype schemas at
   runtime from Rust code (rather than loading pre-authored JSON) must
   construct the JSON manually or depend on TypeBox. Does NOT block any
   current consumer — all v1 consumers (SFTP, metatensor, binary call