Define what counts as an "upload attempt" so counts are trustworthy

[timfong888]

User Story

When we report any upload metric — success rate, durability, failure breakdown —
I want a single, documented definition of what an "upload attempt" is: when it starts, when it ends, and which events mark each outcome,
so every number we publish represents the same activity over the same window, and nobody argues about "what counts."

Context

Without a shared definition, every downstream metric (2-copy rate, store success, pull success) measures something slightly different. A reasonable, accurate count depends on agreeing on the lifecycle boundaries before we instrument anything.

Acceptance Criteria

• "Upload attempt" is defined: what event starts it, what events end it
• Each outcome (2-copy, 1-copy, 0-copy) is mapped to the specific events or conditions that trigger it
• Attempt window / timeout is documented — e.g., how long do we wait for the second copy before classifying as 1-copy?
• Edge cases are covered: client abandons mid-upload, retries, partial commits, out-of-order events
• Definition is reviewed by Curio maintainers and SPs (dealbot owners not required)
• Definition is captured in a durable location (repo doc or design doc) and linked from this Epic

Parent

• [UPLOAD-1] [UPLOAD-1] Know whether uploads are actually achieving 2-copy durability #1