fix: at xs i auto-floors fractional indices

[danieljohnmorris]

Summary

at xs i used to error with ILO-R009 at: index must be an integer whenever i came from arithmetic like / k 2 or any other division. Every persona writing the median-bucket idiom (at xs (len/2)) had to wrap the index in flr (originating entry: html-scraper rerun3 friction #3, ilo_assessment_feedback.md:2542).

Auto-flooring at the at builtin boundary kills the ceremony. The fract!=0 guard is dropped across tree, VM, and Cranelift, and the index is .floor() as i64-ed before the existing negative-index resolution and bounds check. Non-numeric still errors.

Repro

Before:

$ ilo 'f>n;xs=[10,20,30,40,50];k=len xs;i=/ k 2;at xs i' --run-tree f
{"code":"ILO-R009","message":"at: index must be an integer",...}

After:

$ ilo 'f>n;xs=[10,20,30,40,50];k=len xs;i=/ k 2;at xs i' --run-tree f
30

What's in the diff

• interp: drop fract!=0 guard, floor the numeric index, unit tests for positive frac / negative frac / non-numeric error guard.
• vm: same change in OP_AT and jit_at. Replaces the jit_at_fractional_index_returns_nil unit test with three positive-coverage tests (1.7 → 1, -0.5 → -1, -1.5 → -2).
• test: cross-engine regression (tests/regression_at_builtin.rs) covering at xs 1.0, at xs 1.7, at xs -1.5, at xs -0.5, the computed-float len/2 idiom, and a preserved non-numeric error case. Removes the obsolete at_fractional_index_span_matches_vm (jit_at span parity stays covered by the existing OOB tests).
• example: examples/at-float-index.ilo pins the new contract with -- run: / -- out: assertions so examples_engines exercises it across every engine.
• docs: one-line note in SPEC.md, ai.txt, skills/ilo/SKILL.md. Site builtins.md gets a separate PR in ilo-lang/site.

Floor semantics: negative fractions go toward negative infinity, so at xs -0.5 == at xs -1 (last element) and at xs -1.5 == at xs -2 (second-from-last on a len-3 list). Consistent with how flr is documented elsewhere.

Test plan

• cargo test --release --features cranelift clean (all suites)
• regression_at_builtin covers tree / vm / cranelift for: integer index, integral float, positive frac, negative frac, negative half (was the prior error case), computed-float idiom, and non-numeric error preservation
• interp_at_fractional_index_floors + negative-frac + non-numeric variants in the tree-walker
• jit_at_fractional_*_floors x3 in the JIT helper
• examples_engines runs examples/at-float-index.ilo across every engine
• cargo fmt --check clean
• cargo clippy --release --features cranelift --tests -- -D warnings clean

Follow-ups

• Site docs/reference/builtins.md was missing at entirely; row added in a parallel commit to the site repo.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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