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Fix zero-bit left shift and preserve caller stacks - #418
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…to prevent unnecessary altstack usage and added assertion for bits limit.
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A zero-bit
lshift_prevent_overflowmust leave the integer and both caller stacks unchanged. The existing implementation reads intermediate limbs from the altstack even though the zero-iteration helpers never put them there.Integrates trackoor's existing fix from #400, preserving that commit as a merge parent, and adds regressions for U64 and U254 with an empty or populated altstack, zero/one/high-bit/maximum values, and nonzero shifts at 1 and LIMB_SIZE. Caller-owned main-stack and altstack items are checked explicitly.
Validation: both zero-shift regressions fail before the fix; all six
bigint::add::testtests pass after it, including existing addition/doubling tests. Tests used an isolated workspace containing only the unchanged bitvm package and the original locked dependency versions, avoiding unrelated zkVM workspace dependencies. No field or full-workspace tests were run. Formatting and diff checks pass.Fixes #399.