The MT worker previously kept the external-sequence non-empty gate and LDM
invariant assertion in C, bundled with the private CCtx operation. Project the
CCtx pointer and raw-sequence store through a layout-checked seam so Rust owns
the gate and the apply point while C retains only the opaque
ZSTD_referenceExternalSequences() leaf. Preserve the callback ordering after
worker CCtx initialization and keep empty stores as a no-op.
Test Plan:
- git diff --cached --check -- passed before commit.
- Static rg inspection confirmed all projection initializers include the new
sequence-state pointer and no old serialState_applySequences symbol remains.
- Cargo, make, compiler checks, and full tests were not run per worker scope.
ddidderr
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