feat(client): pass through by-handle payload segments

[faresobeid]

What

generate() returns the choice's payload (by-handle segments of a shared file that the prime-rl inference server writes instead of returning routed_experts / sampling_mask inline). Pass-through only. The inline fields and the base64 fast path are unchanged.

Companion PRs: PrimeIntellect-ai/verifiers#2765 (segments on traces) and PrimeIntellect-ai/prime-rl#3902 (writer and consumers; GPU checks there).

Landing order

1. perf(client): read prime-rl's packed completion_logprobs #174 and perf(v1): accept packed CSR sampling masks from prime-rl's /generate verifiers#2778 (packed /generate, with perf(inference): pack /generate per-token payloads and tune API-server GC prime-rl#3900)
2. This PR and feat(v1): by-handle payload segments on traces verifiers#2765
3. feat: pass router-replay ids and sampling masks by handle on multi-node runs prime-rl#3902 (re-pin deps/renderers / deps/verifiers to the merged commits first)
4. Disaggregated P/D: feat(inference): router replay on the V2 runner everywhere prime-rl#3830, fix(pd): honor routed_experts dtype in inline merge router#61 then feat(pd): stitch by-handle routed_experts payload segments router#60

This PR and #170 both add a key after "sampling_mask" in generate()'s result, so the second to land needs a one-line rebase.