fix(test): give the compat library tests the whole compiler command

[lusoris]

Summary

Both sanitizer legs (TSan and ASan+UBSan, run 37760287719) fail on master since #2303 (14dc2972d):

• test_compat_library_gates: test_engine_symbol_does_not_link and test_exported_vmafx_function_links fail.
• test_libvmaf_deprecation: its three compile cases fail.

Every failure prints /usr/bin/ccache: invalid option -- 'a' (or -- 'W'). The cause is not the sanitizers:

• core/test/meson.build passed the tests cc.cmd_array()[0], the first word of the compiler command.
• Those legs build with CC='ccache clang-22', where the first word is the launcher, so the tests ran ccache -shared ....
• Any build with a compiler launcher fails the same way. No library or score is wrong.

The fix:

• Meson passes the whole compiler command, one word per --cc=<word> argument (vmaf_test_cc_args).
• Both tests read it through the new core/test/meson_cc.py.
• The compat AGENTS page records the rule: never cc.cmd_array()[0].

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). See DCO sign-off.
• make format && make lint is green locally: the commit hooks pass (ruff, markdownlint, caveman, generated-index freshness).
• Unit tests pass. CPU build (-Db_lto=false, -j4, warnings as errors):
  • build: 0 warnings;
  • --suite=fast: 410 OK, 0 fail;
  • codegen tests: 132 passed;
  • make test-netflix-golden: 280 passed, 3 skipped;
  • preflight.sh --stage msvcism: pass;
  • affected suites: pass.
• If I touched any SIMD/GPU code path, I ran /cross-backend-diff and the worst ULP is ≤ 2. (not applicable)
• If I touched a feature extractor with SIMD/GPU twins, I either updated every twin or listed the gap under "Known follow-ups" below. (not applicable)
• If I added a new .c / .cpp / .cu / .h / .hpp, it has the appropriate license header (see CONTRIBUTING.md). (meson_cc.py carries the EUPL-1.2 header)
• If this is a breaking change, the commit message uses ! or BREAKING CHANGE: and the migration path is documented below. (not breaking)
• If this PR adds an ADR, the ADR row lives in docs/adr/_index_fragments/<NNNN-slug>.md and nothing else is touched for the index. (no ADR)

Bug-status hygiene (ADR-0165)

• docs/state.md updated in this PR with a row under Recently closed: T-COMPAT-TESTS-CCACHE-FIRST-WORD-2026-10-08.

Netflix golden-data gate (ADR-0024)

• I did not modify any assertAlmostEqual(...) score in the Netflix golden Python tests.
• If I believe a golden value must change, I have explained why below AND pinged @lusoris for a CODEOWNERS exception. (no golden value changes)

Deep-dive deliverables (ADR-0108)

• Research digest — no digest needed: trivial; the tests took one word of a list.
• Decision matrix — no alternatives: only-one-way fix. A shell string with shlex would break on a compiler path with spaces, and skipping under ccache would drop the coverage.
• AGENTS.md invariant note — core/src/AGENTS.d/vmafx-compat.md: compiler-driving tests take the whole command (--cc= words), never cc.cmd_array()[0].
• Reproducer / smoke-test command — pasted below under "Reproducer".
• CHANGELOG fragment — no changelog needed: test harness only.
• Rebase note — no rebase impact: fork-only tests.

Reproducer

# ccache on PATH; the same configuration as the CI sanitizer legs
CC='ccache clang' CXX='ccache clang++' LDFLAGS=-fuse-ld=lld meson setup build core \
  -Db_sanitize=address -Dc_args='-fsanitize=undefined -fno-sanitize=function' \
  -Dcpp_args='-fsanitize=undefined -fno-sanitize=function' --buildtype=debug \
  -Db_lto=false -Db_lundef=false -Denable_cuda=false -Denable_sycl=false
python3 scripts/ci/run_meson_test.py -- -C build test_compat_library_gates test_libvmaf_deprecation

Evidence

Local runs used ccache 4.13.6 (the upstream release binary) and clang 23.1.1, configured like the CI legs.

• Master's two tests, given the first word as master's meson.build passes it, fail with the CI's messages: ccache: invalid option -- 'a' and -- 'W'.
• With this PR, meson test passes in an ASan+UBSan build: test_compat_library_gates, test_libvmaf_deprecation, check_exported_symbols and check_exported_symbols_libvmaf, 4 OK.
• In a TSan build, the two tests are 2 OK.

Known follow-ups

None.