fix(serve): ALiBi slope doc says 2^(-8(h+1)/n); delete the dark layers/a_li_bi.rs (#4700)

[noahgift]

crates/aprender-serve/src/layers/a_li_bi.rs is not compiled. No mod, include! or #[path] names it. Its 354 lines are an older copy of the ScaledRoPE methods plus the ALiBi doc block, and every fn in it has a compiled twin in scaled_rope.rs. Readers who land on it read code that never runs.

Row	Change
1	Delete the uncompiled layers/a_li_bi.rs
2	Correct the ALiBi slope doc in the compiled scaled_rope.rs: for n heads (a power of 2), head h gets 2^(-8(h+1)/n), so head 0 carries 2^(-8/n), never 1.0. That is what ALiBi::compute_slopes computes

Measured on main 812dbe3 plus this diff: cargo fmt --check exits 0, cargo check -p aprender-serve exits 0, cargo test -p aprender-serve --lib alibi passes 38 of 38, and check_src_test_files_wired.sh exits 0.

crates/aprender-serve/.pmat-baseline.json still lists the deleted path as a data row. No build or gate reads that row as a source file, so it is left alone here.

Refs #4700
keep-open: #4700 tracks every dark source file in the tree, and this PR removes one of them.

🤖 Generated with Claude Code

[github-actions]

§13.11 rung 1 — quorum shadow verdict

S13-SHADOW pr=5168 head=46fa943b389a0d8f3039259b8f58d9e18d5f62ed verdict=REFUSE class=Q1 arm_rc=1

Shadow mode: this records a verdict and merges nothing. A refusal
to arm is not a block (§13 adds zero rows to §7) — the pull request is
exactly as green as it was.

[noahgift]

seat 1, la-0.72 APPROVE @ 46fa943

The scaled_rope doc now gives 2^(-8(h+1)/n), which matches compute_slopes in alibi.rs (unchanged). layers/a_li_bi.rs has no mod, include! or path reference and only duplicates an impl fragment, so deleting it is safe. CI green.

Receipt: review-la-0.72-5168-46fa943b38.md

[noahgift]

seat 2, aprender-78 APPROVE @ 46fa943

• The deleted layers/a_li_bi.rs is dark at this head. No include!/include_str!, #[path] or parent-module mod resolves to its exact path (checked by path resolution, not a suffix grep), and no non-Rust file names it.
• The new scaled_rope.rs doc line, slope 2^(-8(h+1)/n), matches the compiled code in layers/alibi.rs (exponent = -((i + 1) as f32) * ratio), so head 0 gets 2^(-8/n).
• CI on this head: 10 success, 2 skipped.