fix(feature): refuse a second float_adm debug instance and file the twins' debug key unsuffixed

[lusoris]

Summary

Closes T-FLOAT-ADM-DEBUG-KEY-UNSUFFIXED-2026-10-01 (maintainer decision Q-015: keep the unsuffixed key, refuse the second instance, align the twins).

• The unsuffixed adm key stays: it is the Netflix golden contract and python/test/ is untouched.
• feature_extractor_vector_append() refuses a second context whose extractor declares the same unsuffixed_debug_key with debug set. vmaf_use_feature() returns -EINVAL and the log names the key and the holder. vmaf --feature float_adm=debug=true --feature float_adm=debug=true:adm_enhn_gain_limit=1.2 now stops with problem loading feature extractor: float_adm after that message, before any frame (it used to fail at frame 0 with "problem reading pictures"). New field VmafFeatureExtractor::unsuffixed_debug_key; debug is read from the parsed options (vmaf_feature_extractor_context_debug_key()).
• The CPU extractor and the CUDA, SYCL, HIP and Metal twins declare the key; the twins list adm_scale0 where they listed adm (as the CPU does), so they file the ratio unsuffixed. Every option case of test_cuda_float_adm_parity and the shared float_adm_twin_parity.h now compares the unsuffixed adm too (18 keys instead of 17).
• ADR-2056 (Q: "Named refusal + align twins (Recommended)"); docs/metrics/adm.md and an AGENTS.d page describe it.
• Tests: test_float_adm_debug_key_refusal (4 cases; fails without the refusal), test_cuda_float_adm_parity 17 of 17 on the RTX 4090 (CUDA build in a scratch dir), fast suite of the CPU build. make test-netflix-golden's pytest set: 280 passed, 3 skipped (run against core/build-golden of this tree).
• Not run: the SYCL, HIP and Metal twins were edited by text (one list entry and one initialiser line each); float_adm_twin_parity.h is compiled by their parity tests, not by anything built here. tidy: cpu core/src/feature/float_adm.c core/src/fex_ctx_vector.cpp core/src/feature/feature_extractor.cpp core/test/test_float_adm_debug_key_refusal.c 0 findings; cuda float_adm_cuda.c + test_cuda_float_adm_parity.c 0; hip float_adm_hip.c 0; sycl float_adm_sycl.cpp 0.

Reproducer

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

Deliverables (ADR-0108)

• Research digest — no digest needed: trivial.
• Decision matrix — ADR-2056 ## Alternatives considered.
• AGENTS.md invariant note — core/src/feature/AGENTS.d/float-adm-debug-key.md.
• Reproducer / smoke-test command — above.
• CHANGELOG fragment — changelog.d/fixed/float-adm-debug-key-refusal.md.
• Rebase note — docs/rebase-notes.md.
• Docs — docs/metrics/adm.md.
• State — row moved to Recently closed.