simd: withdraw mask_gather_chain_u32 (rejected design)

[AdaWorldAPI]

Summary

This PR removes the chained gather that #358 added: mask_gather_chain_u32, mask_gather_chain_u32_under, ChainHop and ChainTest, together with their tests and the parity case 0xD02/0xD03.

The design was wrong, not just slow:

• It treats a hop as a key read. Each anchor row walks every hop, and the address of each read is the value the previous read returned.
• The graph has no foreign keys. Everything is Cartesian coordinates: a hop is a coordinate range on the sealed, ordered address, and a chain of hops is the intersection of those ranges.
• lance-graph D-TELEPORT-0 (#1484) had already measured the gather shape before simd: chained gather — a chain of key reads folded into one pass #358 was written: it speeds up the work but does not eliminate it.

The code before this change is kept on backup/session-011BMJ-2026-10-12. The reason is recorded in .claude/blackboard.md.

This PR adds no replacement. The rebuild reuses lance-graph's mask-risc/Quack folds, and a new primitive lands here only if that rebuild measures a gap.

Test plan

• cargo clippy --all-targets -- -D warnings, AVX-512 tier, env -u RUSTFLAGS
• cargo fmt --check
• cargo test --lib simd_masking_ops: 169 passed
• simd-masking-parity: clippy clean; run reports 16 groups bit-identical
• No other reference to the removed names in src, crates, scripts, .github or docs

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https://claude.ai/code/session_011BMJnDBAfzcuQh5HeT39Rh

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📒 Files selected for processing (4)

• .claude/blackboard.md
• crates/simd-masking-parity/src/lib.rs
• src/simd.rs
• src/simd_masking_ops.rs

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Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-12T22:21:45.528656Z	56c4669	Draft marked ready

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