Clarify `register_with_sampler()` started type and vars

Markup `ctx.started()` type-sig as `set[int]`, rename binding var
`first` to `shm_periods` and add type hints for clarity on context mgr
unpacking.

Also,
- whitespace cleanup: `Type | None` -> `Type|None` throughout
- format long lines: `.setdefault()`, `await ctx.started()`
- fix backtick style in docstrings for consistency
- add placeholder TODO comment for `feed_is_live` check; it might be
  more rigorous to pass the syncing state down thru all this?

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

piker/data/_sampling.py

@@ -80,14 +80,14 @@ class Sampler:
     This non-instantiated type is meant to be a singleton within
     a `samplerd` actor-service spawned once by the user wishing to
     time-step-sample (real-time) quote feeds, see
-    ``.service.maybe_open_samplerd()`` and the below
-    ``register_with_sampler()``.
+    `.service.maybe_open_samplerd()` and the below
+    `register_with_sampler()`.
 
     '''
     service_nursery: None|trio.Nursery = None
 
-    # TODO: we could stick these in a composed type to avoid
-    # angering the "i hate module scoped variables crowd" (yawn).
+    # TODO: we could stick these in a composed type to avoid angering
+    # the "i hate module scoped variables crowd" (yawn).
     ohlcv_shms: dict[float, list[ShmArray]] = {}
 
     # holds one-task-per-sample-period tasks which are spawned as-needed by
@@ -335,7 +335,7 @@ async def register_with_sampler(
     open_index_stream: bool = True,  # open a 2way stream for sample step msgs?
     sub_for_broadcasts: bool = True,  # sampler side to send step updates?
 
-) -> None:
+) -> set[int]:
 
     get_console_log(tractor.current_actor().loglevel)
     incr_was_started: bool = False
@@ -362,7 +362,12 @@ async def register_with_sampler(
 
             # insert the base 1s period (for OHLC style sampling) into
             # the increment buffer set to update and shift every second.
-            if shms_by_period is not None:
+            if (
+                shms_by_period is not None
+                # and
+                # feed_is_live.is_set()
+                # ^TODO? pass it in instead?
+            ):
                 from ._sharedmem import (
                     attach_shm_array,
                     _Token,
@@ -376,12 +381,17 @@ async def register_with_sampler(
                         readonly=False,
                     )
                     shms_by_period[period] = shm
-                    Sampler.ohlcv_shms.setdefault(period, []).append(shm)
+                    Sampler.ohlcv_shms.setdefault(
+                        period,
+                        [],
+                    ).append(shm)
 
                 assert Sampler.ohlcv_shms
 
             # unblock caller
-            await ctx.started(set(Sampler.ohlcv_shms.keys()))
+            await ctx.started(
+                set(Sampler.ohlcv_shms.keys())
+            )
 
             if open_index_stream:
                 try:
@@ -533,6 +543,8 @@ async def open_sample_stream(
     #         yield bistream
     # else:
 
+    ctx: tractor.Context
+    shm_periods: set[int]  # in `int`-seconds
     async with (
         # XXX: this should be singleton on a host,
         # a lone broker-daemon per provider should be
@@ -547,10 +559,10 @@ async def open_sample_stream(
                 'open_index_stream': open_index_stream,
                 'sub_for_broadcasts': sub_for_broadcasts,
             },
-        ) as (ctx, first)
+        ) as (ctx, shm_periods)
     ):
         if ensure_is_active:
-            assert len(first) > 1
+            assert len(shm_periods) > 1
 
         async with (
             ctx.open_stream(