examples: ternlog_popcnt_gap_probe — the mask_ternlog_popcount loop shape is not the gap

[AdaWorldAPI]

This adds examples/ternlog_popcnt_gap_probe.rs, which measures where mask_ternlog_popcount loses time. The answer is that its loop shape is not the gap.

3 planes, (a&b)|c. Every arm is asserted equal. The arms are:

• P: production mask_ternlog_popcount.
• T: the same loop with no popcount, which is the loop's floor.
• M: ternlog plus a Mula popcount (VPSHUFB + VPSADBW), as a lab arm with raw intrinsics.
• S: the plain scalar loop.

Results on a Xeon at 2.8 GHz with AVX-512F/BW and no VPOPCNTDQ:

• P equals M at every size. The scalar-lane popcount fallback compiles as fast as a Mula LUT, so it is not a cost.
• P is within about 1.15× of the floor T. At 1024 words (one 64k-row tile) it is 1.3× faster than S.

5 planes, ((a|b)&c)^(d&!e), 4K–16K words:

• Timings: two-pass 13.3 µs, register-resident two-ternlog 11.8 µs, the same unrolled ×4 11.9 µs, scalar 9.0 µs.
• Fusing the two tables in registers buys only 1.1×.
• The scalar loop is 1.2–1.5× ahead of every U64x8 form.
• Unrolling doesn't explain it, and a rebuild with the 256-bit preference switched on and off was inconclusive.
• The cause is left OPEN; it is small next to the aperture results in lance-graph.

Also fixed: ternlog_fold_probe's header claimed the no-VPOPCNTDQ fallback is a LUT path. It is the scalar-lane fallback, and the header now says so.

Tests: example-only, no library change. cargo clippy --release --example ternlog_popcnt_gap_probe -D warnings is clean.

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📒 Files selected for processing (3)

• Cargo.toml
• examples/ternlog_fold_probe.rs
• examples/ternlog_popcnt_gap_probe.rs

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