Mk `Brokerd[Order].price` avoid `float`-errs

By re-typing to a `.price: Decimal` field on both legs of the EMS.

It seems we must do it ourselves since,
- these msg's (fields) are relayed through the clearing engine to each
  `brokerd` backend and,
- bc many (if not all) of those backends `.broker`-clients (nor their
  encapsulated "brokerage services") **are not** doing any
  precision-truncation themselves.

So, for now, instead we opt to expect rounding at the source. This means
we will explicitly require casting to/from `float` at the line-graphics
interface to the order-clearing-engine (as implemented throughout
`.ui.order_mode.OrderMode`); and this is coming shortly.

piker/clearing/_ems.py

@@ -1182,12 +1182,16 @@ async def process_client_order_cmds(
     submitting live orders immediately if requested by the client.
 
     '''
-    # cmd: dict
+    # TODO, only allow `msgspec.Struct` form!
+    cmd: dict
     async for cmd in client_order_stream:
-        log.info(f'Received order cmd:\n{pformat(cmd)}')
+        log.info(
+            f'Received order cmd:\n'
+            f'{pformat(cmd)}\n'
+        )
 
         # CAWT DAMN we need struct support!
-        oid = str(cmd['oid'])
+        oid: str = str(cmd['oid'])
 
         # register this stream as an active order dialog (msg flow) for
         # this order id such that translated message from the brokerd
@@ -1347,7 +1351,11 @@ async def process_client_order_cmds(
                 # (``translate_and_relay_brokerd_events()`` above) will
                 # handle relaying the ems side responses back to
                 # the client/cmd sender from this request
-                log.info(f'Sending live order to {broker}:\n{pformat(msg)}')
+                log.info(
+                    f'Sending live order to {broker}:\n'
+                    f'{pformat(msg)}'
+                )
+
                 await brokerd_order_stream.send(msg)
 
                 # an immediate response should be ``BrokerdOrderAck``

piker/clearing/_messages.py

@@ -19,6 +19,7 @@ Clearing sub-system message and protocols.
 
 """
 from __future__ import annotations
+from decimal import Decimal
 from typing import (
     Literal,
 )
@@ -71,7 +72,15 @@ class Order(Struct):
     symbol: str  # | MktPair
     account: str  # should we set a default as '' ?
 
-    price: float
+    # https://docs.python.org/3/library/decimal.html#decimal-objects
+    #
+    # ?TODO? decimal usage throughout?
+    # -[ ] possibly leverage the `Encoder(decimal_format='number')`
+    #  bit?
+    # |_https://jcristharif.com/msgspec/supported-types.html#decimal
+    # -[ ] should we also use it for .size?
+    #
+    price: Decimal
     size: float  # -ve is "sell", +ve is "buy"
 
     brokers: list[str] = []
@@ -178,7 +187,7 @@ class BrokerdOrder(Struct):
     time_ns: int
 
     symbol: str  # fqme
-    price: float
+    price: Decimal
     size: float
 
     # TODO: if we instead rely on a +ve/-ve size to determine