fix(server): Claude live command output no longer starts with � after skipping ahead

apps/server/src/orchestration-v2/Adapters/ClaudeTaskOutputTail.test.ts

@@ -91,11 +91,12 @@ describe("tailClaudeTaskOutput", () => {
         }).pipe(Effect.forkScoped);
         assert.equal(yield* Queue.take(chunks), "start\n");
 
-        yield* fileSystem.writeFileString(file, `start\n${"y\n".repeat(1_000_000)}end\n`);
+        // The skip lands on the second byte of a two-byte é.
+        yield* fileSystem.writeFileString(file, `start\n${"é\n".repeat(1_000_000)}end.\n`);
         const flooded = yield* Queue.take(chunks);
         assert.isAtMost(flooded.length, 256 * 1024 + 1);
-        assert.isTrue(flooded.startsWith("\n"));
-        assert.isTrue(flooded.endsWith("y\nend\n"));
+        assert.equal(flooded.slice(0, 3), "\n\né");
+        assert.isTrue(flooded.endsWith("é\nend.\n"));
       }),
     ).pipe(Effect.provide(NodeServices.layer)),
   );

apps/server/src/orchestration-v2/Adapters/ClaudeTaskOutputTail.ts

@@ -104,10 +104,15 @@ export const tailClaudeTaskOutput = Effect.fn("ClaudeTaskOutputTail.tail")(funct
       if (Option.isSome(bytes)) {
         // Skipped output must not splice two unrelated lines together.
         const skipped = start > offset;
-        // Reset partial UTF-8 bytes when skipping output.
-        if (skipped) decoder = new TextDecoder("utf-8", { fatal: false });
+        let first = 0;
+        if (skipped) {
+          // Reset partial UTF-8 bytes, and skip the rest of a code point the cut landed inside.
+          decoder = new TextDecoder("utf-8", { fatal: false });
+          while (((bytes.value[first] ?? 0) & 0xc0) === 0x80) first += 1;
+        }
         offset = start + bytes.value.byteLength;
-        const text = (skipped ? "\n" : "") + decoder.decode(bytes.value, { stream: true });
+        const text =
+          (skipped ? "\n" : "") + decoder.decode(bytes.value.subarray(first), { stream: true });
         if (text.length > 0) yield* input.onChunk(text);
       }
     }