Decode queue compaction receives req_to_token rows after the prefill side has already populated cached prefix slots. Cache-hit requests therefore need the extend/suffix slice, not the leading prefix slice, when building the prebuilt transfer chunk.
Constraint: Prefill/decode disaggregation shares req_to_token rows across cached prefix and new suffix positions.
Rejected: Keep slicing from zero | cache-hit requests would copy prefix KV locs into the prebuilt suffix chunk.
Confidence: medium
Scope-risk: narrow
Directive: Do not change prepare_for_prebuilt slicing without testing cache-hit req_to_token layouts.
Tested: python -m py_compile on changed runtime files.
Not-tested: Local pytest blocked before collection by missing orjson dependency.
(cherry picked from commit 416112b617fabe71e8cff7484794af73f3e84440)
c9a39ccdd2