fix(trace): show span IDs in trace view and fix event_id mapping (#400)

## Summary

`trace view` showed `undefined` for every span ID in the tree because
the trace detail API returns `event_id` instead of `span_id`. This also
broke `span view` lookups since `findSpanById` compared against the
missing field.

Two fixes:
- Normalize the API response in `getDetailedTrace` by copying `event_id`
→ `span_id` when `span_id` is missing
- Append the span ID (dimmed) to each span tree line so users can copy
it into `span view`

Before:
```
├─ http.client — GET /api/users  (52ms)  undefined
```

After:
```
├─ http.client — GET /api/users  (52ms)  a6ff0caaa87dd118
```

## Test plan

- [x] `bun run typecheck` passes
- [x] `bun run lint` passes
- [x] `bun test test/lib/formatters/span-tree.test.ts` — 20 tests pass
- [x] Manual: `trace view <trace-id>` shows real span IDs
- [x] Manual: `span view <span-id> --trace <trace-id>` finds the span

Author: betegon

src/lib/api-client.ts

@@ -90,6 +90,7 @@ export {
 export {
   getDetailedTrace,
   listTransactions,
+  normalizeTraceSpan,
 } from "./api/traces.js";
 
 export {

src/lib/api/traces.ts

@@ -48,7 +48,24 @@ export async function getDetailedTrace(
       },
     }
   );
-  return data;
+  return data.map(normalizeTraceSpan);
+}
+
+/**
+ * The trace detail API (`/trace/{id}/`) returns each span's unique identifier
+ * as `event_id` rather than `span_id`. The value is the same 16-hex-char span
+ * ID that `parent_span_id` references on child spans. We copy it to `span_id`
+ * so the rest of the codebase can use a single, predictable field name.
+ */
+export function normalizeTraceSpan(span: TraceSpan): TraceSpan {
+  const normalized = { ...span };
+  if (!normalized.span_id && normalized.event_id) {
+    normalized.span_id = normalized.event_id;
+  }
+  if (normalized.children) {
+    normalized.children = normalized.children.map(normalizeTraceSpan);
+  }
+  return normalized;
 }
 
 /** Fields to request from the transactions API */

src/lib/formatters/human.ts

@@ -1122,6 +1122,8 @@ function formatSpanSimple(span: TraceSpan, opts: FormatSpanOptions): void {
     line += `  ${muted(`(${prettyMs(durationMs)})`)}`;
   }
 
+  line += `  ${muted(span.span_id ?? "")}`;
+
   lines.push(line);
 
   if (currentDepth < maxDepth) {

test/lib/api-client.normalize-trace-span.test.ts

@@ -0,0 +1,30 @@
+import { describe, expect, test } from "bun:test";
+import { normalizeTraceSpan } from "../../src/lib/api-client.js";
+
+describe("normalizeTraceSpan", () => {
+  test("copies event_id to span_id when span_id is missing", () => {
+    const span = { event_id: "abc123", start_timestamp: 0 } as Parameters<
+      typeof normalizeTraceSpan
+    >[0];
+    const result = normalizeTraceSpan(span);
+    expect(result.span_id).toBe("abc123");
+  });
+
+  test("preserves existing span_id", () => {
+    const result = normalizeTraceSpan({
+      span_id: "existing",
+      event_id: "other",
+      start_timestamp: 0,
+    });
+    expect(result.span_id).toBe("existing");
+  });
+
+  test("recurses into children", () => {
+    const result = normalizeTraceSpan({
+      span_id: "parent",
+      start_timestamp: 0,
+      children: [{ event_id: "child1", start_timestamp: 1 } as any],
+    });
+    expect(result.children?.[0]?.span_id).toBe("child1");
+  });
+});