ffc_sv_create: zero just the length

[kolemannix]

The array shouldn't need to be zero-initialized.
We should never read from any element of the array that has not been initialized by a push or extend

[Copilot]

Pull request overview

This PR optimizes ffc_sv_create by removing the zero-initialization of the entire ffc_sv struct (which includes a large stack-allocated array of up to 125/62 limbs) and instead only initializing the len field to 0. This is safe because the data array elements are always written before being read (via ffc_sv_try_extend), and this pattern is already used elsewhere in the codebase.

Changes:

• Replaced ffc_sv new_one = {0} with ffc_sv new_one; new_one.len = 0; in ffc_sv_create, avoiding unnecessary zeroing of the data array.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/bigint.h	Remove zero-initialization of ffc_sv in ffc_sv_create, only set len = 0
ffc.h	Mirror the same change in the amalgamated single-header version

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[kolemannix]

Ran the exhaustive suite as well for this guy:

~/dev/ffc.h no_zero_bigint_create 17s ❯ FFC_EXH_THREADS=10 make -B exhaustive                                                12:54:44 PM
python3 amalgamate.py > ffc.h
mkdir -p out
clang -xc -Wall -Wextra -Wpedantic -O3 -g -std=c99  -pthread -I. -Itest_src test_src/test_exhaustive.c -o out/test_exhaustive_runner -lm
./out/test_exhaustive_runner
Running exhaustive test: threads=10 sample_stride=1 (~100.00% of 4294967296 values)
Progress: 10% (429496730/4294967296)
Progress: 20% (858993460/4294967296)
Progress: 30% (1288490189/4294967296)
Progress: 40% (1717986919/4294967296)
Progress: 50% (2147483648/4294967296)
Progress: 60% (2576980378/4294967296)
Progress: 70% (3006477108/4294967296)
Progress: 80% (3435973837/4294967296)
Progress: 90% (3865470567/4294967296)
Progress: 100% (4294967296/4294967296)
Exhaustive run completed: checked 4294967296 values in 2211.51 seconds
all ok