fix(code-runtime-python): close coverage gap and tighten the wire mirror

- Cover the log-frame `truncated` rebuild branch: assert a literal-true flag
  rides along and any other value (1, string, false) is dropped, closing the
  protocol.ts branch the coverage gate flagged.
- Correct encodeJsonPlain's JSDoc: it matches compact JSON.stringify EXCEPT on
  a beyond-safe-range integral double, where it emits the exact BigInt digits
  (`...846976`) rather than the rounded `...847000` — the divergence the
  "emits exact digits" test pins.
- Declare py/protocol.py's `global`-bearing frames (Namespace, CallMessage)
  with functional TypedDict syntax so they carry the real wire key instead of
  a `global_` attribute the wire never sends, and split optional-field messages
  (Namespace/LogMessage/DoneMessage) into a required base plus a total=False
  subclass so `type` and other required fields cannot be dropped. Widen
  HostToChild to include the boot and run frames the host sends before replies.
- Reword the mirror e2e's py/ directory assertion to describe the source-tree
  layout it actually checks.

packages/code-runtime/code-runtime-python/py/protocol.py

@@ -3,6 +3,13 @@
 Mirrors ``src/protocol.ts``. Frames travel on fd 3 as JSON-lines (one JSON
 object per line). The host validates every inbound frame; this side trusts
 host replies.
+
+The wire uses the JSON key ``global`` (a Python keyword), so the frame
+``TypedDict``s that carry it are declared with the functional syntax rather than
+class bodies: a class attribute cannot be named ``global``, and a ``global_``
+attribute would describe a key the wire never sends. Optional-field messages
+pair a required base with a ``total=False`` subclass so a required field such as
+``type`` cannot be dropped while ``value``/``error``/``truncated`` stay optional.
 """
 
 from __future__ import annotations
@@ -15,17 +22,6 @@ from typing import Any, Literal, TypedDict, Union
 PROTOCOL_FD = 3
 
 
-class BootMessage(TypedDict):
-    """Host → child, first frame on fd 3. Carries every cap and the namespaces."""
-
-    type: Literal["boot"]
-    cpuSeconds: int
-    addressSpaceBytes: int
-    maxLogBytes: int
-    maxValueBytes: int
-    namespaces: list["Namespace"]
-
-
 class ErrorClass(TypedDict):
     """A namespace's program-visible exception class: rejected calls raise its
     instances carrying the failed member name on ``memberNameProperty``."""
@@ -34,13 +30,27 @@ class ErrorClass(TypedDict):
     memberNameProperty: str
 
 
-class Namespace(TypedDict, total=False):
-    """One binding namespace declaration: the global name, its function names,
-    and an optional program-visible ``errorClass`` for rejected calls."""
+# ``global`` is a Python keyword, so the required part is declared functionally
+# to hold the real wire key; ``errorClass`` is optional per the TS `errorClass?`.
+_NamespaceRequired = TypedDict("_NamespaceRequired", {"global": str, "names": "list[str]"})
 
-    global_: str  # required; renamed on the wire: JSON field is ``global`` (Python keyword collision)
-    names: list[str]  # required
-    errorClass: ErrorClass  # optional — mirrors the TS `errorClass?`
+
+class Namespace(_NamespaceRequired, total=False):
+    """One binding namespace declaration: the ``global`` name, its function
+    ``names``, and an optional program-visible ``errorClass`` for rejected calls."""
+
+    errorClass: ErrorClass
+
+
+class BootMessage(TypedDict):
+    """Host → child, first frame on fd 3. Carries every cap and the namespaces."""
+
+    type: Literal["boot"]
+    cpuSeconds: int
+    addressSpaceBytes: int
+    maxLogBytes: int
+    maxValueBytes: int
+    namespaces: "list[Namespace]"
 
 
 class RunMessage(TypedDict):
@@ -56,17 +66,17 @@ class BootAckMessage(TypedDict):
     type: Literal["boot-ack"]
 
 
-class CallMessage(TypedDict):
-    """Child → host: one bridged binding call from the model program."""
-
-    type: Literal["call"]
-    id: int
-    global_: str  # wire field is ``global``
-    name: str
-    args: Any
+# ``global`` wire key: whole message declared functionally, all fields required.
+CallMessage = TypedDict(
+    "CallMessage",
+    {"type": Literal["call"], "id": int, "global": str, "name": str, "args": Any},
+)
 
 
-class LogMessage(TypedDict, total=False):
+_LogMessageRequired = TypedDict("_LogMessageRequired", {"type": Literal["log"], "text": str})
+
+
+class LogMessage(_LogMessageRequired, total=False):
     """Child → host: one captured text chunk, streamed eagerly.
 
     ``truncated`` is set only on the frame that IS the child ledger's truncation
@@ -74,9 +84,7 @@ class LogMessage(TypedDict, total=False):
     the child did — mirrors the TS `truncated?`.
     """
 
-    type: Literal["log"]  # required
-    text: str  # required
-    truncated: bool  # optional
+    truncated: bool
 
 
 class DoneErrorField(TypedDict):
@@ -87,10 +95,12 @@ class DoneErrorField(TypedDict):
     message: str
 
 
-class DoneMessage(TypedDict, total=False):
+_DoneMessageRequired = TypedDict("_DoneMessageRequired", {"type": Literal["done"]})
+
+
+class DoneMessage(_DoneMessageRequired, total=False):
     """Child → host: the program settled. ``value`` and ``error`` are optional per the TS mirror."""
 
-    type: Literal["done"]  # required — TypedDict(total=False) allows this via a required subclass in Py 3.11+; MVP keeps it flat
     value: Any
     error: DoneErrorField
 
@@ -113,7 +123,9 @@ class ReplyErr(TypedDict):
 
 
 ReplyMessage = Union[ReplyOk, ReplyErr]
-HostToChild = ReplyMessage
+# The host sends ``boot`` and ``run`` before any ``reply``, so the child-facing
+# inbound union covers all three, not replies alone.
+HostToChild = Union[BootMessage, RunMessage, ReplyMessage]
 
 
 def log_truncation_marker(max_bytes: int) -> str: