fix(code-runtime-python): merge a flushed unterminated line into the next log entry

The review's remaining warning: an explicit flush of an unterminated line
(print(..., end='', flush=True)) pushed a full log frame, so the following
print() landed in a second entry and logs.join('\n') rendered 'a\nb' for what
the program printed as one line — a model-visible output defect. The flush
frame now carries an  flag (LogMessage gains the optional field on both
sides and in the mirror test), the host holds it and appends the next log frame
to the same entry, and finish() admits the residual if the run ends with it
still open. The settlement note registers the decimal-context fix from the
previous commit.

packages/code-runtime/code-runtime-python/src/protocol.ts

@@ -102,6 +102,13 @@ interface LogMessage {
    * and keeps exactly one marker in `logs`.
    */
   truncated?: boolean
+  /**
+   * Set on the frame an explicit `flush()` (or the settlement flush) pushes for
+   * an UNTERMINATED line: the host holds it and appends the next log frame to
+   * the same entry, so `print('a', end='', flush=True); print('b')` reads back
+   * as one `'ab'` entry rather than a fake newline between two entries.
+   */
+  open?: boolean
 }
 
 /** The failure carried on a {@link DoneMessage}: one of three kinds plus text. */
@@ -227,7 +234,7 @@ const WIRE_FRAME_FIELD_ROLES = {
   RunMessage: { type: 'required', program: 'required' },
   BootAckMessage: { type: 'required' },
   CallMessage: { type: 'required', id: 'required', global: 'required', name: 'required', args: 'required' },
-  LogMessage: { type: 'required', text: 'required', truncated: 'optional' },
+  LogMessage: { type: 'required', text: 'required', truncated: 'optional', open: 'optional' },
   DoneErrorField: { kind: 'required', message: 'required' },
   DoneMessage: { type: 'required', value: 'optional', error: 'optional' },
   ErrorClass: { name: 'required', memberNameProperty: 'required' },
@@ -611,8 +618,13 @@ export function validateChildFrame(raw: unknown): ChildToHost | undefined {
       if (typeof m.text !== 'string') return undefined
       // Rebuilt, not passed through: a forged `truncated` of any other type
       // would reach the host as a truthy value and silence capture for the
-      // rest of the run. Only the literal `true` counts.
-      return { type: 'log', text: m.text, ...m.truncated === true ? { truncated: true } : {} }
+      // rest of the run. Only the literal `true` counts; `open` likewise.
+      return {
+        type: 'log',
+        text: m.text,
+        ...m.truncated === true ? { truncated: true } : {},
+        ...m.open === true ? { open: true } : {},
+      }
     case 'call': {
       // The id must be a finite number: it is echoed verbatim into the reply
       // frame, and a forged `1e400` id (Infinity after JSON.parse) would make