Remove unused code from the post NNUE accumulator merge

[zungur]

After merging the HalfKA and Threats accumulators (7c7fe32) and the subsequent removal of the double-incremental/fused update, a number of NNUE helpers and fields became unreachable. Each was verified to have zero callers/readers across the source tree:

• FusedUpdateData logic in FullThreats: the fused-update branch of append_changed_indices and the FusedUpdateData parameter are unused; the accumulator update no longer passes fused data.
• FullThreats::requires_refresh: never called. The live king-bucket refresh check is HalfKAv2_hm::requires_refresh (PSQFeatureSet), used in nnue_accumulator.
• HalfKAv2_hm::append_active_indices: never called. The live active-index builder is FullThreats::append_active_indices (ThreatFeatureSet).
• DirtyThreats::us, prevKsq and ksq: written in do_move but only read by the now-removed FullThreats::requires_refresh. Removing them also drops three stores from the do_move path.
• Unused feature Name constants and the unused FtOneVal / HiddenMaxVal constants in nnue_common.h.
• Two stale feature-header banner comments.

No functional change

[github-actions]

clang-format 20 needs to be run on this PR.
If you do not have clang-format installed, the maintainer will run it when merging.
For the exact version please see https://packages.ubuntu.com/plucky/clang-format-20.

(execution 27386387097 / attempt 1)

[coderabbitai]

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Configuration used: Organization UI

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📥 Commits

Reviewing files that changed from the base of the PR and between 3c15e6f and b9aa732.

📒 Files selected for processing (8)

• src/nnue/features/full_threats.cpp
• src/nnue/features/full_threats.h
• src/nnue/features/half_ka_v2_hm.cpp
• src/nnue/features/half_ka_v2_hm.h
• src/nnue/nnue_accumulator.cpp
• src/nnue/nnue_common.h
• src/position.cpp
• src/types.h

💤 Files with no reviewable changes (4)

• src/types.h
• src/nnue/nnue_common.h
• src/position.cpp
• src/nnue/features/full_threats.cpp

🚧 Files skipped from review as they are similar to previous changes (3)

• src/nnue/nnue_accumulator.cpp
• src/nnue/features/half_ka_v2_hm.h
• src/nnue/features/full_threats.h

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📝 Walkthrough

Walkthrough

This PR refactors NNUE threat feature handling to remove fused-update infrastructure and simplify state tracking. The DirtyThreats struct is reduced to contain only a list field, removing caller-supplied metadata. FullThreats removes its FusedUpdateData helper type and simplifies the append_changed_indices signature by removing fusedData and first parameters. HalfKAv2_hm removes active-index scanning and aligns its API accordingly. Call sites in nnue_accumulator are updated to use the simplified signature. Unused constants FtOneVal and HiddenMaxVal are removed.

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[github-actions]

clang-format 20 needs to be run on this PR.
If you do not have clang-format installed, the maintainer will run it when merging.
For the exact version please see https://packages.ubuntu.com/plucky/clang-format-20.

(execution 27387019374 / attempt 1)