Coalesce the final prefill chunk of off-grid prompts and share steps with decodes

[original-el8]

Purpose

Two follow-ups to the continuation-coalescing plan (local-inference-lab#699), rebased onto this branch's current head (5115b91).

1. prefill_checkpoint_plan accepts a final span that ends at the prompt end off the 512-token grid; continuation_layout derives the final state column from the last token. A prompt whose length is not a block multiple previously never coalesced its last span and fell into the reachable-boundary ladder: 16,228 tokens ran as 8192, 4096, 2048, 1024, 512, 356 with one retained checkpoint per chunk. It now runs as 8192 + 8036 with the interior retained states exported in one pass; the final state lands in the tail block exactly as an ordinary unaligned final chunk stores it.
2. Coalescing is admitted when at most one request still has prompt tokens to compute, instead of when the engine holds exactly one request. Plans are per request and the worker maps them by packed row, so decoding requests sharing the step do not interfere. Two prefills in flight keep the ordinary splitting as before.

Evidence (four DGX Sparks, GLM-5.3-Flash-NVFP4-Spark, TP4/DCP4, MTP3, 8,192 budget, 512-token blocks, 18 GiB KV)

Cold prefill, fixed prompts, six samples per length, A/B/A/B alternating launches (baseline = this branch's coalescing without the change):

Prompt tokens	Baseline TTFT (means)	With change	Change
16,228	5.668 s	4.831 s	-14.8%
16,383	5.621 s	5.308 s	-5.6%
32,319	10.354 s	10.020 s	-3.2%
8,192 / 16,384 / 32,768 (aligned controls)	2.440 / 5.001 / 9.863 s	2.421 / 4.826 / 9.830 s	-0.8% / -3.5% / -0.3%

Cold prefill beside three decoding 8K requests, two samples per arm, two alternating pairs: 8,192 tokens 3.413 s -> 2.709 s (-20.6%), 16,228 tokens 6.022 s -> 5.313 s (-11.8%). Every arm's answer and prefix-cache checks passed. The rank-0 log shows the new coalesced spans (span=8036, 7743, 8191, four checkpoints each).

A third variant that retreated the span to the last block boundary when decode budget tokens push the end off the grid measured slower than these two commits and is not included.

Tests

tests/v1/core/test_recurrent_prefill_checkpoint.py: new cases for the unaligned final plan, its allocator columns at DCP4 (16,228 / 16,383 / 32,319 / 6,244 prompts), the single-token tail, and prompt-only exclusivity; 60 passed on this head (CPU). tests/models/test_glm5next_pooled_indexer.py and the RoCE suite passed on all four ranks in-image.

Residual: a final span whose row count is not a multiple of four (16,383 -> 8,191 rows) misses mHC row ownership, which is why that length gains less.

AI assistance was used for implementation, testing and evidence preparation.

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