fix(rc): implement picture_v2 API + fix ai/scripts stub exit codes

[lusoris]

Summary

• VmafPicture v2 fully implemented (ADR-0928 cycle N+1): all five entry points in core/src/picture_v2.c are now live in libvmaf.so. Previously the header was design-only and all symbols were unlinked. Header now installed via meson install_headers. 10-case unit test added to the fast suite (test_picture_v2).
• 13 ai/scripts stub scripts fixed: all exited with code 0 (success) when not yet implemented, deceiving CI and any caller into believing work had completed. All now exit with code 1 and print a stderr guidance message pointing to the relevant doc/ADR.
• No new stubs remain misleading: the remaining -ENOSYS patterns (HIP/Metal/DNN disabled-build stubs, MCP scaffold) are correct conditional-build behavior, not misleading silences.

Scaffold inventory

Item	Status	Action
picture_v2.h — 5 unlinked entry points	Completed	Implemented in picture_v2.c, header installed
13 ai/scripts/*.py sys.exit(0) stubs	Tombstoned properly	Changed to sys.exit(1)
HIP/Metal -ENOSYS stubs	Correct behavior	Left as-is (documented scaffold contract)
DNN disabled-build stubs	Correct behavior	Left as-is (ADR-0040/ADR-0374)
MCP scaffold	Already implemented (T5-2b)	Left as-is

Test plan

• meson test -C <build> --suite=fast → 89/89 pass (up from 88)
• test_picture_v2 covers: vmaf_backend_handle_name known + OOB, vmaf_picture2_alloc/unref happy + null paths, vmaf_picture_v1_to_v2/v2_to_v1 happy + null paths, round-trip
• All 13 stub scripts now exit 1 when invoked (verified via shell)
• Pre-commit passes (clang-format, ruff, black, copyright, semgrep)

Deliverables checklist

• Research digest: no digest needed: trivial (implementation is mechanical wrapping of existing picture.c)
• Decision matrix: no alternatives: only-one-way fix (the stubs had to be either implemented or removed; implemented because the design was correct)
• AGENTS.md invariant: updated core/include/libvmaf/AGENTS.md picture_v2 invariant note
• Reproducer: meson test -C build --suite=fast (test_picture_v2 at index 17)
• Changelog fragment: changelog.d/fixed/rc-scaffold-stub-completion.md
• Rebase notes: docs/rebase-notes.md § rc/scaffold-stub-completion

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