Narrow ITensors serialization public API to just the schema struct types

Drops `serialized_type`, `serialize_convert`, and `deserialize_convert`
from the `public` declaration. These remain reachable from the JLD2
extension (and from `Base.convert` via the shims) but are no longer part
of the package's stable public surface — no current external consumer
calls them directly, so committing to them now is premature.

The JLD2 file format docs page is updated to match: the prose describes
the internal layering without rendering docstring blocks for the
unexported helpers, so the `@docs` API list is limited to the schema
struct types themselves.

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

Author: mtfishman

docs/src/JLD2FileFormat.md

@@ -47,22 +47,16 @@ Keeping the structs and the conversion logic in the main packages means the type
 recorded inside the JLD2 file do not encode an extension module namespace, and other
 serialization backends can reuse the same machinery without depending on JLD2.
 
-### Backend-agnostic layer
-
-  - [`ITensors.serialized_type`](@ref) and [`NDTensors.serialized_type`](@ref): type-level
-    map from an in-memory type to its [`SerializedX`](@ref) schema struct.
-  - `Base.convert` overloads (defined alongside `serialized_type`) do the value-level
-    transform between the in-memory and `Serialized*` representations in both directions.
-
-JLD2's default `wconvert` / `rconvert` already delegate to `Base.convert`, so the
-extension itself only needs to provide the `JLD2.writeas` declarations bridging the
-two namespaces:
-
-```julia
-JLD2.writeas(::Type{T}) where {T <: NDTensors.TensorStorage} = NDTensors.serialized_type(T)
-JLD2.writeas(::Type{T}) where {T <: ITensors.Index}          = ITensors.serialized_type(T)
-# ...
-```
+Internally the extension is structured around two unexported helper functions per
+package — a type-level `serialized_type` that maps an in-memory type to its
+schema struct, and a value-level pair `serialize_convert` / `deserialize_convert`
+that owns the field-by-field transform. `Base.convert` overloads delegate to
+these helpers so non-JLD2 callers can also reach the schema, and the JLD2
+extension itself only needs to register a few `JLD2.writeas` declarations
+plus permissive `JLD2.rconvert` overloads to bridge into JLD2's machinery.
+These helpers are public-but-internal: they may evolve and are not part of the
+stable public API surface — only the [`SerializedX`](@ref) schema structs
+themselves are.
 
 ### Core ITensors types
 

src/ITensors.jl

@@ -89,12 +89,9 @@ include("set_operations.jl")
 include("indexset.jl")
 include("itensor.jl")
 include("serialization.jl")
-# Mark the on-disk schema types and the serialization conversion entry points as
-# public API (per Julia docs recommendation for backwards-compatible `public`
-# declarations on pre-1.11 Julia).
 VERSION >= v"1.11.0-DEV.469" && eval(
     Meta.parse(
-        "public serialized_type, serialize_convert, deserialize_convert, SerializedQNVal, SerializedQN, SerializedTagSet, SerializedQNSpace, SerializedIndex, SerializedITensor"
+        "public SerializedQNVal, SerializedQN, SerializedTagSet, SerializedQNSpace, SerializedIndex, SerializedITensor"
     )
 )
 include("qn/flux.jl")