fix(cli): say "problem scoring picture" when libvmaf fails to score a frame

[lusoris]

Summary

Closes T-CLI-EXTRACTOR-ERROR-PROPAGATION-2026-10-05. When vmaf_read_pictures() failed (an extractor refusing a frame or its options, e.g. float_ms_ssim on a 16x16 picture or integer ADM refusing a viewing geometry), the CLI printed problem reading pictures, one word away from the input read failure it prints as problem while reading pictures (exit 102). The pictures had been read; libvmaf could not score them.

• core/tools/vmaf.cpp prints problem scoring picture N: libvmaf returned E through a small helper, after the libvmaf message that names the extractor. The exit status (the libvmaf code, 234 for -EINVAL) and the read-failure wording and exit 102 are unchanged.
• core/tools/test/test_vmaf_score_error_message.sh (suite fast, positive / negative / boundary): a refused frame gives exit 234 and problem scoring picture 0, neither old wording appears, and a truncated input still gives exit 102 with problem while reading pictures. On the previous binary it fails with problem reading pictures.
• docs/usage/cli.md exit-code table lists the case; the SYCL Windows and NR-metric docs quote the new line.
• tidy: cpu core/tools/vmaf.cpp 0 findings (scripts/dev/tidy-lane.sh --only core/tools/vmaf.cpp --jobs 4 cpu, clang-tidy 22.1.8); scripts/dev/preflight.sh --stage msvcism passes.

Reproducer

meson setup core/build-cpu core -Denable_cuda=false -Denable_sycl=false && ninja -C core/build-cpu tools/vmaf
meson test -C core/build-cpu test_vmaf_score_error_message test_vmaf_read_error_exit

Deliverables (ADR-0108)

• Research digest — no digest needed: trivial.
• Decision matrix — no alternatives: only-one-way fix (the row asks the CLI to tell the two cases apart; the exit status is kept for scripts).
• AGENTS.md invariant note — no rebase-sensitive invariants.
• Reproducer / smoke-test command — above.
• CHANGELOG fragment — changelog.d/fixed/cli-scoring-error-message.md.
• Rebase note — no rebase impact: one fork-edited line of vmaf.cpp's frame loop, covered by the new test.
• Docs — docs/usage/cli.md, docs/backends/sycl/windows.md, docs/ai/models/nr_metric_v1.md.
• State — row moved to Recently closed in docs/state.md.