fix(test): zero-initialise the SAD tables of the motion-window stream tests

[lusoris]

Summary

The Cppcheck job of the Lint workflow fails on master since #2290 (e584c79a6). cppcheck 2.19.0, the Ubuntu 26.04 package CI installs, reports uninitvar on sad in core/test/test_motion_window_incremental.c at lines 256 and 289.

Verdict: false positive. Both loops write every entry that mwi_values_match() reads:

• In mwi_stream() the writes go through the permuted index mwi_arrival(k, n, shuffled). That order is a permutation of 0..n-1 for every stream length the test uses (0, 1, 2, 3, 4, 5, 9 and 17; checked).
• In test_flush_without_state_equals_upstream_flush() the writes are in order.

Each loop stops early only on an error, and the read is guarded by !err. cppcheck cannot see coverage through the permuted index.

Fix: both tables are zero-initialised before their loops, so they are whole on every path. A comment names the permutation. There is no suppression.

#2645 carries the same change from before #2290 landed. It has been blocked on a conflict in this file since 00:34. I have not closed it or marked it superseded; that is for its owner to decide.

Type

• fix — bug fix
• test — test-only

Checklist

• Commits follow Conventional Commits (the commit-msg hook enforces this).
• Every commit is signed off (git commit -s; fix a branch with git rebase --signoff origin/master). See DCO sign-off.
• clang-format --dry-run --Werror is clean. cppcheck 2.19.0 with the CI flags is clean on the file.
• Unit tests pass: test_motion_window_incremental (5 tests) and test_motion_window_advance_contract.

Bug-status hygiene (ADR-0165)

• docs/state.md updated in this PR with a row in Recently closed (T-CPPCHECK-MOTION-WINDOW-SAD-UNINITVAR-2026-10-09).

Netflix golden-data gate (ADR-0024)

• I did not modify any assertAlmostEqual(...) score in the Netflix golden Python tests.

Deep-dive deliverables (ADR-0108)

• Research digest — no digest needed: one static-analysis false positive, explained above.
• Decision matrix — no alternatives: cppcheck cannot follow a permuted index, and a suppression is ruled out.
• AGENTS.md invariant note — no rebase-sensitive invariants: test-only change.
• Reproducer / smoke-test command — see "Reproducer" below.
• CHANGELOG fragment — no changelog needed: test-only change.
• Rebase note — no rebase impact: fork-only test.

Reproducer

meson setup build core -Denable_cuda=false -Denable_sycl=false
python3 scripts/ci/write-compile-commands.py --build-dir build
docker run --rm -v "$PWD:$PWD" -w "$PWD" ubuntu:26.04 bash -c '
  apt-get update -qq && apt-get install -y -qq cppcheck python3 >/dev/null
  python3 scripts/ci/write_cppcheck_posix_model.py --cppcheck cppcheck --output build/cppcheck-posix-vmafx.cfg
  cppcheck --enable=warning,performance,portability --check-level=exhaustive --inline-suppr \
    --library=build/cppcheck-posix-vmafx.cfg --library=scripts/ci/cppcheck-public-entrypoints.cfg \
    --suppressions-list=.cppcheck-suppressions.txt --project=build/compile_commands.json \
    --file-filter="*core/test/test_motion_window_incremental.c" --error-exitcode=1 -q'
# origin/master: uninitvar at :256:55 and :289:70, exit 1.   This branch: no output, exit 0.
python3 scripts/ci/run_meson_test.py -- -C build test_motion_window_incremental test_motion_window_advance_contract

Known follow-ups

None.