parser: chain 2-arg numeric builtins without parens

[danieljohnmorris]

Summary

Closes pending #5ao. abs rndn 0 1 (and any chain putting a 2-arg numeric builtin under a unary outer) was misparsing as abs(rndn, 0, 1) and erroring with "arity mismatch: abs expects 1 args, got 3". Workaround had been abs (rndn 0 1) — agents kept reaching for parens that the manifesto wants to avoid.

Root cause: parse_call_arg only recurses into a nested call when the inner ident has a registered arity in builtin_arity_tables(). rndn, atan2, fmod, clamp, and the unary trig forms (tan, asin, acos, atan, log10, log2) were missing from that table, so the parser fell through to parse_operand, returned a bare Ref(rndn), and let the outer greedy loop swallow the remaining operands.

The structural fix is to register the missing arities. The nested-call branch already exists and is the right code path; this just lets it fire for these builtins exactly like it already does for pow and min/max.

Repro

Before:

$ ilo 'g>n;abs rndn 0 0.1' g
{"code":"ILO-T004","message":"undefined variable 'rndn'", ...}
{"code":"ILO-T006","message":"arity mismatch: 'abs' expects 1 args, got 3", ...}

After:

$ ilo 'g>n;abs rndn 0 0.1' g
0.04951257174436721

What's in the diff

• parser: register arity for missing numeric builtins — adds the missing entries to builtin_arity_tables(). Lowest-blast-radius fix that takes the existing nested-call branch.
• test: cover chained 2-arg numeric calls cross-engine — 9 regression tests across --vm and --jit covering abs/sqrt/pow outer × atan2/fmod/clamp/rndn/log10 inner, the pending repro shape (y=+1 rndn 0 0.1), and a composed pow atan2 1 1 2. Adds examples/chained-num-args.ilo for agent in-context learning, exercised by examples_engines.rs.
• docs: spell out known-arity call chaining rule — SPEC.md Call Arguments section and ai.txt CALLS line now document the chain-without-parens behaviour, with the variadic/unknown-arity carve-out called out.

Test plan

• cargo test --release --features cranelift passes (all 3273 lib + every integration suite green)
• cargo test --release --features cranelift --test regression_chained_1arg_hint — 9/9 pass
• cargo test --release --features cranelift --test examples_engines — new example exercised
• cargo fmt --check clean
• cargo clippy --release --features cranelift --all-targets -- -D warnings clean
• Manual: abs rndn 0 0.1, sqrt atan2 1 1, abs clamp 5 0 10, pow atan2 1 1 2 all run on both engines

Follow-ups

There are ~60 more builtins still missing from the parser arity table (incl. take, drop, enumerate, chunks, window, quantile, median, stdev, variance, matmul, dot, transpose, inv, det, solve, zip). Each has the same latent chained-call trap. This PR fixes the cluster the pending entry called out; happy to widen in a follow-up once we've watched main bake.

[codecov]

Codecov Report

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

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