Qualify slow upload, sustained throughput, and resource bounds

[niteshpurohit]

Objective

Prove file staging remains bounded and responsive under sustained/hostile upload workloads.

Acceptance Criteria

Cover:

• slow multipart metadata;
• slow file bodies;
• abandoned uploads;
• many-file requests;
• concurrent uploads;
• sustained streaming throughput;
• staging disk backpressure;
• active byte/file ceilings;
• reaper under load;
• CPU/RSS/fd/allocation/syscall/tail-latency measurements;
• control/admin reserve remains usable;
• no secret, physical path, or unbounded opaque-ID leakage.