refactor(qwen35): split scheduler step drivers into scheduler/{steps,emit,telemetry}.rs

[CAICAIIs]

What

Continue the scheduler.rs slim-down along the seams backend.rs/tp.rs (#967, #968) established: step drivers, request-emission concerns, and ITL telemetry move out of scheduler/mod.rs, which keeps the request/artifact types, the entry point, load publishing, and the main loop (1896 → 882 lines). Behavior unchanged.

Stacked PR 3/3 — based on #1035 + the responses-split PR; Files-changed includes both until they merge. Merge those first. Rebuilt over #1004/#1005/#1033/#1038 (the #1033 policy-aware start stays in the scheduler entry).

Structure

• scheduler/steps.rs (846 lines) — batch prefill, overlap launch, unified prefill+decode step, pure decode, token dispatch/retirement, DecodeDispatchBackend/PrefillPromoteBackend, and the chunk vocabulary (ScheduledChunk, InflightPrefill, take_prefill_chunks, promote_or_requeue) shared with the backend.
• scheduler/emit.rs (126 lines) — terminal shutdown fan-out, closed-request pruning, unsupported-echo/rejection paths.
• scheduler/telemetry.rs (45 lines) — the PEGAINFER_ITL_DEBUG per-step ITL trio.

Moved items gain pub(super) only where the loop, backend, tp, or emit siblings name them; step-internal helpers (decode_step_with_seed, process_decode_logits, compact_single_slot, fail_chunk, prefill_lifecycle_failure, split_scheduled_backend_state) stay private to steps.rs.

Defense table (all inherited — pure move)

Defense	Failure mode	Successor
Terminal shutdown fans every owned request an error	requests stranded on scheduler exit	inherited: emit.rs terminal_scheduler_shutdown
Async-prefill stream drained before KV/recurrent release	use-after-free of GPU state on shutdown	inherited: emit.rs drop-order comment + steps.rs InflightPrefill
Closed-request pruning before admission	zombie requests occupy slots/pages	inherited: emit.rs prune_closed_requests
echo=true rejected (fail-closed, not degraded)	unsupported serving contract silently accepted	inherited: emit.rs reject_unsupported_echo
Start-gate cancel on partial dispatch	one rank mutated while a peer skipped the step	inherited: steps.rs gate checks + decode_step family
ITL telemetry env-gated (off = zero cost)	bench path pays logging overhead	inherited: telemetry.rs itl_debug_enabled OnceLock

Validation

• cargo fmt --check clean; cargo clippy -p pegainfer-qwen35 --features qwen35 --all-targets -- -D warnings clean.
• cargo test --lib green; tp2_* GPU tests pass on 2×A100 (-- --ignored --test-threads=1); e2e_scheduler green.

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[chatgpt-codex-connector]

Revert unrelated TP phase wording cleanup

This behavior-preserving scheduler/TP module split also rewrites phase terminology here and in lib.rs, even though those edits do not support either file-movement invariant. Keeping unrelated documentation and user-facing error-message changes in this refactor broadens its review surface and makes the claimed no-behavior-change scope harder to verify; revert them or move them to a separately justified change.

AGENTS.md reference: AGENTS.md:L121-L123

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