feature: asin/acos/atan inverse trig builtins

[danieljohnmorris]

Summary

Geospatial personas (gis-analyst rerun4) keep hand-rolling a 6-term Taylor series for asin because the language ships sin/cos/tan/atan2 but no inverse trig. That's a token tax on every haversine and bearing computation, and a silent precision footgun. Rust's f64 already has asin, acos, atan natively, so the fix is plumbing across the three engines.

Manifesto framing: builtins are dedicated opcodes, so adding them removes Taylor expansions from agent output forever and saves tokens every time someone reaches for great-circle distance.

Repro

Before:

-- hand-rolled asin (the persona's actual code)
asin x:n>n;t1=x;x3=*x *x x;t2=/x3 6;x5=*x3 *x x;t3=*/x5 40 3;...

After:

hav lat1:n lon1:n lat2:n lon2:n>n;
  p1=*lat1 0.017453292519943295;
  ...
  s=sqrt a;
  as=asin s;
  *6371 *2 as

hav 51.5074 -0.1278 48.8566 2.3522 returns 343.55606034104164 on all engines.

What's in the diff

• add asin/acos/atan inverse trig builtins (tree interpreter): registers the variants and wires the tree interpreter.
• vm: add OP_ASIN/OP_ACOS/OP_ATAN and jit_* helper externs: new opcodes 171/172/173 + JIT helper functions following the existing NaN-on-non-number pattern.
• cranelift: wire asin/acos/atan helpers in JIT and AOT codegen: declares the FuncIds, registers the symbols, extends the unary-math arm in both JIT and AOT paths.
• test: cross-engine regression + haversine example for inverse trig: 16 cross-engine assertions in tests/regression_inverse_trig.rs plus examples/inverse-trig-haversine.ilo which tests/examples_engines.rs runs end-to-end.
• docs: document asin/acos/atan in spec, ai.txt and SKILL.md: keeps the four canonical docs in sync.

Test plan

• cargo test --release --features cranelift --test regression_inverse_trig — 16 passed
• cargo test --release --features cranelift --test examples_engines — haversine example passes on tree + vm
• cargo test --release --features cranelift full suite green locally (the only failures were libilo.a lookups that resolve when the static lib is in target/release/; CI builds at that path)
• cargo fmt --check
• cargo clippy --release --features cranelift --all-targets -- -D warnings

Follow-ups

None blocking. Worth considering whether to also expose pi as a constant so *180 /x pi reads cleanly without the magic 0.0174... literal, but that's a separate ask.

[codecov]

Codecov Report

❌ Patch coverage is 88.52459% with 7 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/vm/compile_cranelift.rs	50.00%	4 Missing ⚠️
src/vm/mod.rs	88.46%	3 Missing ⚠️

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