Expect registry of fsp "flows" to each engine task

In order for fsp routines to be able to look up other "flows" in the
cascade, we need a small registry-table which gives access to a map of
a source stream + an fsp -> an output stream. Eventually we'll also
likely want a dependency (injection) mechanism so that any fsp demanded
can either be dynamically allocated or at the least waited upon before
a consumer tries to access it.

piker/fsp/_engine.py

@@ -37,8 +37,11 @@ from .. import data
 from ..data import attach_shm_array
 from ..data.feed import Feed
 from ..data._sharedmem import ShmArray
-from ._api import Fsp
+from ._api import (
-from ._api import _load_builtins
+    Fsp,
+    _load_builtins,
+    _Token,
+)
 
 log = get_logger(__name__)
 
@@ -99,6 +102,8 @@ async def fsp_compute(
         # to the async iterable? it's that or we do some kinda
         # async itertools style?
         filter_quotes_by_sym(symbol, quote_stream),
+
+        # XXX: currently the ``ohlcv`` arg
         feed.shm,
     )
 
@@ -239,6 +244,8 @@ async def cascade(
 
     ns_path: NamespacePath,
 
+    shm_registry: dict[str, _Token],
+
     zero_on_step: bool = False,
     loglevel: Optional[str] = None,
 
@@ -261,9 +268,21 @@ async def cascade(
     log.info(
         f'Registered FSP set:\n{lines}'
     )
-    func: Fsp = reg.get(
+
+    # update actor local flows table which registers
+    # readonly "instances" of this fsp for symbol/source
+    # so that consumer fsps can look it up by source + fsp.
+    # TODO: ugh i hate this wind/unwind to list over the wire
+    # but not sure how else to do it.
+    for (token, fsp_name, dst_token) in shm_registry:
+        Fsp._flow_registry[
+            (_Token.from_msg(token), fsp_name)
+        ] = _Token.from_msg(dst_token)
+
+    fsp: Fsp = reg.get(
         NamespacePath(ns_path)
     )
+    func = fsp.func
 
     if not func:
         # TODO: assume it's a func target path