cli: engine flags share non-ident positional -> main heuristic with default

[danieljohnmorris]

Summary

PR #329 propagated half of the default-engine entry-point heuristic to the explicit-engine flags (--run-tree / --run-vm / --run-cranelift): when there's no positional after the source path, auto-pick main. It didn't propagate PR #328's other half: when the first positional doesn't look like a function name (paper.txt, /tmp/x.json, 42, anything with a . or / or leading digit) and main is defined, route to main with the positional as arg #1.

Three rerun7 personas (pdf-analyst, devops-sre, qa-tester) hit the same asymmetry independently:

$ ilo main.ilo paper.txt                # default engine: works
42
$ ilo --run-tree main.ilo paper.txt     # tree: fails
{"code":"ILO-R002","message":"undefined function: paper.txt", ...}
$ ilo --run-vm main.ilo /tmp/x.json     # vm: same
$ ilo --run-cranelift main.ilo paper.txt # cranelift: same

This PR extends resolve_engine_func_name (src/main.rs) to share the same looks_like_subcommand_name check the default-engine path uses. Surgical fix, mirrors a check that's already documented and well-tested elsewhere.

Repro before/after

/tmp/repro.ilo:

helper x:n>n;+x 1
main path:t>n;helper 41

Before:

$ ilo --run-tree /tmp/repro.ilo /tmp/paper.txt
{"code":"ILO-R002","message":"undefined function: /tmp/paper.txt", ...}
$ ilo --run-vm /tmp/repro.ilo /tmp/paper.txt
{"code":"ILO-R002","message":"undefined function: /tmp/paper.txt", ...}
$ ilo --run-cranelift /tmp/repro.ilo /tmp/paper.txt
undefined function: /tmp/paper.txt

After:

$ ilo --run-tree /tmp/repro.ilo /tmp/paper.txt        → 42
$ ilo --run-vm /tmp/repro.ilo /tmp/paper.txt          → 42
$ ilo --run-cranelift /tmp/repro.ilo /tmp/paper.txt   → 42
$ ilo /tmp/repro.ilo /tmp/paper.txt                   → 42  (unchanged)

What's in the diff

• cli: engine flags route non-ident first positional to main (1cd81db) — the four-line guard in resolve_engine_func_name. Identical predicate to the default-engine branch at src/main.rs ~2838-2857, so future drift between the two paths is easier to spot.
• tests: cross-engine regression for non-ident positional dispatch (89bed33) — 24 new tests in regression_cli_default.rs mirroring the PR cli: auto-pick main on --run-tree / --run-vm / --run-cranelift #329 block: one helper per scenario, fanned out across --run-tree / --run-vm / --run-cranelift plus a default-engine pin. Covers non-ident positional, path-shaped positional, numeric positional, explicit main keyword + non-ident arg, explicit helper override (must NOT be shadowed by the heuristic), and ident-shaped-unknown-no-main (must still error).
• examples: pin non-ident positional routing across engines (8b04f63) — examples/engine-flag-non-ident-positional.ilo runs through the engine harness on every backend, so a future regression here fails at cross-engine parity rather than waiting for another persona to file it.

Test plan

• cargo test --release --features cranelift (full suite green, all 3000+ tests)
• cargo fmt --check clean
• cargo clippy --release --features cranelift --tests -- -D warnings clean
• Manual repro across all four engines confirms 42
• Examples harness exercises paper.txt, /tmp/x.json, explicit main, and explicit helper on every backend

Follow-ups

• qa-tester rerun7 inline variant (ilo --run-tree 'f a:n b:n>n;/a b' 10 0 with one fn, no main) is NOT covered here. The narrow brief was "share cli: hyphenated subcommands + non-ident positionals route to main (#320 follow-up) #328's non-ident -> main heuristic", and that case has no main to route to. Worth a follow-up to consider a single-fn inline auto-run on the explicit-engine paths, but it's a wider design call than this PR.

[codecov]

Codecov Report

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

📢 Thoughts on this report? Let us know!