verify: warn when recursive self-call return is discarded

[danieljohnmorris]

Summary

Adds ILO-T043, a verifier warning that fires when a recursive self-call sits at a non-tail position and its return value is silently discarded. Closes the diagnostic gap surfaced 2026-05-21 by the interp1d persona via #5ba investigation (PR #571): the persona wrote a braceless-guard recursive search where the recursive call was followed by a literal fallback, every call discarded the recursive return, and the function always returned the fallback. With no warning emitted, the persona then mis-diagnosed the bug as broken braceless guards.

T043 is narrowly scoped to recursive self-calls (callee name == caller name) with a non-nil declared return type. Bare non-recursive user-fn calls at non-tail position do NOT warn, because the callee may legitimately be side-effecting (logging, file I/O). The narrow surface keeps the false-positive risk near zero; we can broaden later if reruns surface other classes.

Manifesto framing: a missing signal here is paying for itself in agent confusion tokens (the persona spent an entire investigation chasing a phantom braceless-guard bug). Adding the signal once means every future agent who reaches for the same shape gets pointed at the real footgun immediately.

Repro before/after

Before:

find-idx xs:L n target:n i:n>n;>=i len xs (-1);=target at xs i i;find-idx xs target +i 1;-1

ilo check is silent. Function always returns -1 because the recursive call's return is dropped.

After:

warning[ILO-T043]: recursive call to 'find-idx' is at a non-tail position and its return value is discarded
  hint: a call is only in tail position when it's the last statement of the body, an arm of a tail match,
        or the body of a braceless guard. To use the recursive result, restructure as `?h cond {result}
        {fallback}` or `=cond {recursive-call}`. To return early, write `ret <call>`.

Fix shape (examples/recursive-tail-position.ilo):

find-idx xs:L n target:n i:n>n;>=i len xs (-1);=target at xs i i;find-idx xs target +i 1

Recursive call is now the body's last statement (tail position); braceless guards short-circuit above.

What's in the diff

Per-commit:

1. verify: warn on non-tail recursive self-call return discard - new ILO-T043 check in verify_body, narrowly scoped to recursive self-calls with non-nil return.
2. test: regression for ILO-T043 recursive discard - cross-engine regression at tests/regression_verify_discarded_recursive.rs (5 cases: warns on non-tail self-call, no-warn on tail, no-warn on non-recursive, no-warn on ret-wrapped, multiple warns).
3. docs: ILO-T043 registry entry + example + SPEC sync - --explain ILO-T043 now resolves; SPEC.md tail-position section points at the new code; examples/recursive-tail-position.ilo pins the canonical fix shape across every engine via examples_engines.

Test plan

• cargo test --release --features cranelift --test regression_verify_discarded_recursive (5/5 passing)
• examples/recursive-tail-position.ilo main returns 3, missing returns -1 (manual run)
• Full suite: cargo test --release --features cranelift green
• CI green (main currently red due to PR feat(http): getx/pstx with status + headers + body Ok-map (#5bn) — fresh reimpl #566 token-budget + crypto-primitives panic; only checking my changes don't introduce new failures)

Follow-ups

• Broaden to all user-fn discards if rerun data surfaces other false-positive classes.
• Consider extending to detect =cond recursive-call shapes (assignment in a non-tail position with a recursive RHS) - currently Stmt::Let isn't covered.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
7574	1	7573	0

View the top 1 failed test(s) by shortest run time

ilo::skill_md::body_is_thin_bootstrap

Stack Traces | 0.005s run time

thread 'body_is_thin_bootstrap' (65093) panicked at tests/skill_md.rs:254:5:
SKILL.md body is 10925 bytes; bootstrap shape should stay well under 8 KB
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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