perf(dflash): split-chain fast rollback with F32 SSM checkpoints (+29% decode)

[Amidwestnoob]

Summary

• add layer-split F32 SSM checkpoint capture and fast pure-chain rollback for DFLASH
• keep production tree verification fail-closed and reject sibling/non-chain geometry before execution
• add a CPU-only root-inclusive pure-chain guard regression test

Benchmark

Controlled dual-GPU A/B/A benchmark on two RTX 3090 24 GB cards using the same pinned models, prompt, generation length, power/thermal conditions, and layer split (--target-gpus 0,1 --target-layer-split 12,11). Each block used one discarded warmup and five measured runs.

Metric	Baseline	Candidate	Delta
Median decode throughput	22.76 tok/s	29.41 tok/s	+29.22%
Median wall time	24.895 s	22.560 s	-9.38%
Prompt processing	~95.4 tok/s	~95.4 tok/s	flat
Acceptance rate	42.4%	37.2%	-5.2 pp
Average commit	7.76	5.95	-1.81

All 15 measured runs produced byte-identical 256-token output:

1653a412811d3b87e26aab900e801b411fd68f9214b512ae984db6404b82204f

A1/A2 baseline drift was 0.48%. Maximum observed GPU temperature was 64 C.

Memory trade-off

The F32 SSM checkpoint storage adds exactly 1,774,190,592 bytes (~1.65 GiB) of persistent GPU memory in the tested split configuration. Both GPUs are required for the tested 27B configuration; a single RTX 3090 is not sufficient.

Safety boundary

• supports_tree_verify() remains unconditionally false in production.
• Only the validated root-inclusive pure-chain rollback path is enabled.
• Sibling, malformed-root, skipped-parent, cyclic/forward-parent, out-of-range, truncated, and null geometry is rejected before the execution sentinel.
• Fast rollback is opt-in through DFLASH_SPLIT_FAST_ROLLBACK and requires successful capture validation.
• Temporary evidence instrumentation is excluded from this commit.

Verification

• rebased onto current upstream main at 5e302cbb483819cd21e72f5dd8becaa609eca8cf
• preserved upstream fix(qwen35): route non-tree GDN capture through persist buffer (src[7]) #469 non-tree GDN persist-buffer routing through src[7]
• full Release CUDA 8.6 build and link: passed
• current upstream test suite: 15/15 passed, including CUDA and the pure-chain guard
• rebased-candidate dual-GPU validation: passed; 43/43 fast rollbacks succeeded with zero fallback
• rebased validation produced the same generated suffix hash as the controlled benchmark
• controlled A/B/A benchmark: 18/18 isolated processes passed, including warmups
• no OOM, CUDA, fallback, tree-path, output-parity, or thermal failures

Scope

Measured on the exact dual-RTX-3090 configuration above with a 1,024-token prompt and 256 generated tokens. No performance claim is made for single-GPU execution, sibling-tree verification, other models, or other hardware.

[cubic-dev-ai]

All reported issues were addressed across 13 files

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[davide221]

@Amidwestnoob thanks for the contribution, please review and fix cubic reviews