Module start 

The start surface: options, outcome, and the SDK-boundary idempotency key. The start surface of the caller SDK: options, outcome, the SDK-boundary idempotency fingerprint, and Client::start itself.

Split out of ops.rs (which the rest of the operations still share) when the display-name work (#211) pushed that file past the house 500-code-line limit. The seam is cohesive rather than arbitrary: everything here answers one question — what a start request is, what comes back from it, and when two starts are the same act.

Structs

DisplayNameNotApplied

The report that an idempotent start replay did not apply the display name this call requested (#211).

StartOptions

Options accepted by Client::start.

StartOutcome

What Client::start returns: the started (or idempotently replayed) run, plus the note that a requested display name was NOT applied.