[wieslawsoltes]

FileReader split every Blob read across two native tasks, so protocol consumers paid two scheduler and microtask/style checkpoints per message. Complete the asynchronous FileReader lifecycle in one browser task while preserving the required loadstart → Promise continuation → progress → load/error → loadend ordering.

This advances #280 and #252. It removes the measured FileReader bottleneck; the remaining product bound is independently tracked by #289.

Validation:

• pinned Node 24.18.1 compatibility tests: 8/8 pass
• focused native websocket-file-reader gate: pass, including 100 socket cycles and 20 protocol cycles
• FileReader p95: 0.02525 ms; 64-timer backlog: 0.314 ms; queue high-water: 1
• exact unchanged Code OSS package: five provider children, five Explorer model children, five visible rows, and 64 native nodes pass
• product FileReader maximum improved from 1,114 ms to 31–45 ms across candidate runs
• best exact first-child paint improved from 2,530 ms to 2,043 ms; Bound the packaged remote extension-host Ready-to-Initialized handshake #289 owns the remaining remote WebSocket Ready delay

Detailed evidence: docs/validation/file-reader-single-task-20260921.md.