Add step claim attribution to client spans (#4066)

## Summary & Motivation

Adds three bounded trace attributes so a sampled trace says how a `step_started` claim was made: `workflow.step_start.strategy` on the step span (`awaited` / `optimistic` / `batch_preclaimed`, set before the write so a losing claim keeps it after its 409 reconciles to `skipped`), and `workflow.step_start.mode` plus `workflow.step_start.owner_stamped` on the world-vercel write spans across the http, batch, and ws paths. Purely additive telemetry — no change to execution behavior.

## Test Plan

Test added covering a stamped lazy claim's attributes on the per-write span; existing coverage runs in CI. Local run of 81 focused core and world-vercel tests passed; full core typecheck is blocked by unrelated workspace resolution issues.