A tool call can say it failed, and what it is for, without a renderer

P1b's pure half (docs/RUST.md). Three pieces, all testable with no
widget in sight:

- `event_model::Event::ToolEnd` gains `is_error`, read from the CLI's own
  `tool_result` field by both the live translator and the import replay
  (`import::tool_result_is_error`, one reader so the two cannot disagree
  about the same conversation). Without it a result is all a card has,
  and a broken call draws exactly as confidently as one that worked --
  the missing state, not a wrong one. `#[serde(default)]`, so an older
  transcript reads back as "not reported to have failed".
- `client_core::transcript_fold::ToolState`: Running, Deciding,
  Succeeded, Failed, NoResult. The pair it exists for is the last two
  against Succeeded-with-empty-output -- a call that printed nothing and
  a call whose result never arrived leave the same empty string, and only
  the session's status separates "still going" from "nobody found out".
- `client_core::tool_summary::parse_tool_input` and
  `client_core::durations`: `ToolInput.kt`'s subject/description/timeout
  split and `Durations.kt`'s span formatting, ported with their tests.

The echo driver's three-call run now has a failing middle call, so the
failed appearance is reachable from `ui-sandbox.sh` at all.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

server/src/session/import.rs

@@ -363,6 +363,22 @@ fn is_hidden(record: &Value) -> bool {
         || record.get("isMeta").and_then(Value::as_bool) == Some(true)
 }
 
+/// Whether a `tool_result` block says the call itself failed.
+///
+/// One reader for the field rather than one per caller: the live
+/// translator (`translate.rs`) and this replay of the CLI's own file look
+/// at the same block shape, and a call drawn as failed in one and as
+/// succeeded in the other would be the same conversation disagreeing with
+/// itself. Absent means "not reported to have failed" -- which is what the
+/// CLI writes for a call that went fine, and also what every transcript
+/// written before this field was read says.
+pub(crate) fn tool_result_is_error(block: &Value) -> bool {
+    block
+        .get("is_error")
+        .and_then(Value::as_bool)
+        .unwrap_or(false)
+}
+
 fn text_of(content: &Value) -> String {
     match content {
         Value::String(text) => text.clone(),
@@ -549,6 +565,7 @@ fn push_user(events: &mut Vec<Event>, content: &Value, session_dir: &std::path::
                 events.push(Event::ToolEnd {
                     id: id.to_string(),
                     output: text_of(block.get("content").unwrap_or(&Value::Null)),
+                    is_error: tool_result_is_error(block),
                 });
             }
         }