Moved derive units to registry architectur

DeriveUnits are now owned by a DeriveRegistry which has its own routing
thread to handle routing values to the different DeriveUnits

docs/API.md

@@ -123,7 +123,8 @@ representation.
 ### `get_all_signal_descriptors() -> list[SignalDescriptor]`
 
 Returns descriptors for every signal currently registered by connected
-connectors. The list is empty when no connectors have completed a handshake.
+connectors and bound derive units. The list is empty when no connectors have
+completed a handshake and no derive units are bound.
 
 Connector membership is dynamic: a disconnected connector's signals are no
 longer returned.
@@ -233,9 +234,9 @@ The parameter annotations must use the imported `ValueDescriptor` and
 `SignalDescriptor` classes shown above. The source string is executed with
 `exec()`, so only bind code from trusted sources.
 
-The returned UUID identifies the bound unit. Binding invalid source or more
-than one function raises `CoreMultipleFunctionsFoundError`; an invalid function
-signature raises `DeriveUnitInvalidSignatureError`.
+The returned UUID identifies the bound unit. Binding source that defines more
+or fewer than one function raises `DeriveUnitMultipleFunctionsFoundError`; an
+invalid function signature raises `DeriveUnitInvalidSignatureError`.
 
 ### `unbind_derive_unit(unit_id: UUID) -> None`
 
@@ -252,5 +253,5 @@ Core-specific exceptions are defined in `dynalab_core.errors`.
 | Exception | Raised when |
 | --- | --- |
 | `CoreStateMismatchError` | `start()` is called after the core has already started or stopped. |
-| `CoreMultipleFunctionsFoundError` | A derive-unit source string does not define exactly one function. |
+| `DeriveUnitMultipleFunctionsFoundError` | A derive-unit source string does not define exactly one function. |
 | `DeriveUnitInvalidSignatureError` | A derive function's annotations or parameter order are invalid. |