clippy debt ratchet 1/5 (0.70): apr-cli + aprender-core + aprender-serve residual (~522) #4153

Clippy tech-debt ratchet, slice for 0.70.0. Filed by the cop (aprender-cf) on the operator's instruction, verbatim, 2026-09-24: "move those issues into our next 5 release technical debt rachet and split evenly".

Baseline: measured by aprender-30 (#4152) on main aa7c6ef with cargo clippy --workspace --all-targets --no-deps -- -D warnings (GPU trio excluded, as CI does): 3807 diagnostics in 15 crates. #4152 removes the 1280 aprender-serve excessive_precision findings (mirroring the workspace allow) and installs the member-wide ratchet (keyed on crate × target-kind × lint; only decreases allowed). That leaves 2527, split into five roughly equal slices over 0.70–0.74.

This slice (0.70.0): apr-cli (284) + aprender-core (218) + aprender-serve residual after #4152 (~20).

Done when:

• every clippy finding in this slice is fixed in code. The unwrap / expect bans are fixed with ?, ok_or_else or a documented expect where the invariant is proven, never with #[allow];
• the clippy -D warnings fails on aprender-serve on main (1280 excessive_precision in IQ tables, 5 deny-level undocumented unsafe) — no gate lints it: Makefile clippy is facade-only and serve doesn't inherit workspace lints #4152 ratchet baseline shrinks by the fixed count in the same PR;
• the ratchet total is ≤ 2005 after this release;
• no new ratchet key appears.

Sequence: 0.70 ≤2005 → 0.71 ≤1445 → 0.72 ≤886 → 0.73 ≤376 → 0.74 = 0. Depends on #4152.

Collapsed rows (APR-EPIC-001 rule 20)

One issue per PR: tick a line when its PR merges; the PR closing the last line says Closes #4153, the others Refs #4153 row <id>.

• clippy debt ratchet 2/5 (0.71): qa-runner, first half (~560) #4154: clippy debt ratchet 2/5 (0.71): qa-runner, first half (~560) (clippy debt ratchet 2/5 (0.71): qa-runner, first half (~560) #4154)
• clippy debt ratchet 3/5 (0.72): qa-runner, second half (~559) #4155: clippy debt ratchet 3/5 (0.72): qa-runner, second half (~559) (clippy debt ratchet 3/5 (0.72): qa-runner, second half (~559) #4155)
• clippy debt ratchet 4/5 (0.73): test-cli + half the small crates (~510) #4156: clippy debt ratchet 4/5 (0.73): test-cli + half the small crates (~510) (clippy debt ratchet 4/5 (0.73): test-cli + half the small crates (~510) #4156)
• clippy debt ratchet 5/5 (0.74): aprender-graph + remaining small crates (~376) → 0 #4157: clippy debt ratchet 5/5 (0.74): aprender-graph + remaining small crates (~376) → 0 (clippy debt ratchet 5/5 (0.74): aprender-graph + remaining small crates (~376) → 0 #4157)