fix(compaction): validate retry_in backoff before rendering the retry note

updateCompactionProgress coerced evt.retry_in with Number() and rendered it
unguarded, while the sibling part/total path two lines below is finiteness-
validated — a malformed backoff would render "retrying in NaNs". Validate
retry_in the same way (finite, non-negative), and keep the error text
regardless: the error is the load-bearing half of the note, so an unparseable
duration drops to "retrying (error)…" rather than suppressing the whole arm.

Addresses PR review feedback on the compaction reducer.

tests/test_conversation_js.py

@@ -168,3 +168,17 @@ def test_unbounded_render_inputs_are_capped() -> None:
     assert "more preview lines not shown" in body
     assert "RAW_CAP" in body
     assert "truncated for display" in body
+
+
+def test_retry_note_validates_backoff_before_rendering() -> None:
+    """retry_in is a server-emitted backoff coerced with Number(); like the
+    part/total pair just below it, it must be finiteness-validated (and
+    non-negative) so a malformed value can't render "retrying in NaNs".  The
+    error text is kept regardless — it is the load-bearing half of the note."""
+    body = _body()
+    assert "Number.isFinite(secs) && secs >= 0" in body, (
+        "retry_in must be validated (finite, non-negative) before its seconds render"
+    )
+    assert "Math.round(Number(evt.retry_in))" not in body, (
+        "retry_in must not be Math.round(Number(...))'d without a finiteness guard"
+    )

turnstone/shared_static/conversation.js

@@ -169,12 +169,16 @@ export function updateCompactionProgress(el, evt) {
     return;
   }
   if (evt.retry_in != null) {
+    // retry_in is a server-emitted backoff (seconds) coerced with Number();
+    // validate it the way part/total below are, so a malformed value can't
+    // render "retrying in NaNs".  The error text is the load-bearing half —
+    // keep it whether or not the duration parses.
+    const secs = Number(evt.retry_in);
+    const err = String(evt.error || "error");
     note.textContent =
-      "retrying in " +
-      Math.round(Number(evt.retry_in)) +
-      "s (" +
-      String(evt.error || "error") +
-      ")…";
+      Number.isFinite(secs) && secs >= 0
+        ? "retrying in " + Math.round(secs) + "s (" + err + ")…"
+        : "retrying (" + err + ")…";
     return;
   }
   const part = Number(evt.part);