fix(test): include integer_ssim.h unconditionally in i686 no-asm build

[lusoris]

Summary

• test_integer_ssim_simd.c uses integer_ssim_moments_t in the unconditional scalar reference functions (scalar_accumulate_row_8, scalar_accumulate_row_16) but the only include that brings in that type is gated behind #if ARCH_X86
• On i686 no-asm builds (--cross-file=build-aux/i686-linux-gnu.ini -Denable_asm=false), ARCH_X86 is NOT defined in config.h — src/meson.build only sets it inside if is_asm_enabled
• Result: two error: unknown type name 'integer_ssim_moments_t' compile errors; CI job "Build — Ubuntu i686 gcc (CPU, no-asm)" has failed since PR feat(simd): AVX2 integer SSIM horizontal moment accumulation #145 merged
• Fix: add an unconditional #include "integer_ssim.h" (the shared type header introduced by ADR-1040 / PR fix(build): restore integer_ssim_moments_t type definition (macOS Clang unblock) #654) before the arch-gated AVX2 include

The AVX2 function calls remain inside #if ARCH_X86 so the linker does not need those symbols on no-asm builds.

Reproducer

meson setup build-i686 --cross-file=build-aux/i686-linux-gnu.ini -Denable_asm=false
ninja -C build-i686 test/test_integer_ssim_simd
# Before: test_integer_ssim_simd.c:102: error: unknown type name 'integer_ssim_moments_t'
# After: builds cleanly

Root cause chain

1. PR feat(simd): AVX2 integer SSIM horizontal moment accumulation #145 — introduced integer_ssim_moments_t in x86/integer_ssim_avx2.h and the test
2. PR fix(build): restore integer_ssim_moments_t type definition (macOS Clang unblock) #654 (ADR-1040) — promoted the typedef to integer_ssim.h for macOS arm64, fixed integer_ssim.c, but missed test_integer_ssim_simd.c
3. This PR — fixes the test file

Deliverables checklist

• research digest: no digest needed: trivial missing-include fix
• decision matrix: no alternatives: only-one-way fix (add the missing include)
• AGENTS.md invariant note: no rebase-sensitive invariants
• reproducer: meson setup build-i686 --cross-file=build-aux/i686-linux-gnu.ini -Denable_asm=false && ninja -C build-i686 test/test_integer_ssim_simd
• changelog fragment: see below
• rebase-notes.md: no rebase impact: test-only single-line include fix

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