cli: restore documented auto-run for main and inline programs

[danieljohnmorris]

Summary

Two soundness-class regressions in the CLI default-engine dispatch:

1. ilo file.ilo with main-as-default broken. A multi-fn file containing main was erroring with defines multiple functions, please specify one instead of auto-running main. SKILL.md ([CLI invocation]) has documented this contract since 0.x.

2. ilo 'f>n;42' silently AST-dumping. Inline programs with a runnable entry were dumping AST JSON to stdout with exit 0 instead of running. For piped consumers this is the worst kind of bug: valid-looking JSON output that is not the program result. SKILL.md: "Inline programs (ilo 'code') and single-function files run their entry function with the remaining CLI args; no explicit function name needed."

Plus a related papercut several persona reports flagged: synthetic __lit_N inline-lambda decls were leaking into the "available functions" listing.

The CLI default branch handled the single-fn-inline / multi-fn / main cases inconsistently with the documented contract, treating inline as "always AST-dump" and treating multi-fn files as "always error". This PR aligns the runtime with the docs and tightens the dispatcher to hide compiler-internal names.

Repro

Before:

$ cat > /tmp/automain.ilo <<EOF
> helper a:n>n;+a 1
> main>n;helper 41
> EOF
$ ilo /tmp/automain.ilo
error: /tmp/automain.ilo defines multiple functions, please specify one.
available functions:
  helper
  main
$ ilo 'f>n;42'
{
  "declarations": [
    { "Function": { "name": "f", ... } }
  ]
}
$ echo $?
0

After:

$ ilo /tmp/automain.ilo
42
$ ilo 'f>n;42'
42

What's in the diff

Five discrete commits:

1. cli: hide synthetic __lit_N decls from func-name discovery — filter Decl::Function whose name starts with __ at the source. Synthetic names from inline-lambda lifting (parser/mod.rs ~line 2863) no longer leak into the auto-run heuristic, the multi-fn listing, or CLI positional-arg matching. __ is reserved for the compiler so the filter is safe and forward-compatible if more synthetic prefixes land.
2. cli: prefer main when a multi-fn file omits the func arg — restore the documented main-as-default convention for multi-fn files. Multi-fn files without main keep the friendly listing.
3. cli: auto-run inline programs with a runnable entry — ilo '<single fn>' and ilo '<multi-fn with main>' now auto-run. Zero-fn (declaration-only / comment-only) and multi-fn-without-main inline snippets still AST-dump, preserving the inspection escape hatch from PR main: skip verify errors in inline AST-dump mode #178. --ast remains the explicit form for any case where AST output is wanted regardless of shape.
4. test: pin auto-run contract across file and inline paths — extends tests/regression_cli_default.rs with 7 new cross-cutting tests, adds examples/autorun-main.ilo so the engine harness exercises the main-as-default path across tree / VM / Cranelift, and so future agents have an in-context demonstration.
5. test: migrate pre-existing inline-snippet tests to --ast — several tests in eval_inline.rs, regression_friendly_errors.rs, regression_p001_cascade.rs, and main.rs's embedded test module used inline single-fn snippets with required params as parse-acceptance smoke tests, relying on the implicit AST-dump path. Switched to explicit --ast so the parse-acceptance signal stays decoupled from runtime semantics.

Test plan

• cargo test --release --features cranelift — 5296 passed, 0 failed across 134 suites
• cargo fmt --check
• cargo clippy --release --features cranelift --all-targets -- -D warnings
• New regressions: multi_fn_file_with_main_auto_runs_main, multi_fn_file_explicit_func_arg_overrides_main, inline_single_fn_auto_runs, inline_multi_fn_with_main_auto_runs_main, inline_multi_fn_without_main_still_dumps_ast, inline_zero_fn_still_dumps_ast, synthetic_lambda_decls_hidden_from_multi_fn_listing
• examples/autorun-main.ilo runs across every engine via tests/examples_engines.rs
• Manual repro verified fixed for both reported shapes
• Existing legacy contracts still pinned: single-fn auto-run, multi-fn-without-main listing, --ast always wins, comment-only inline AST-dumps

Follow-ups

None. SKILL.md was already correct; the bug was runtime drift from the documented contract.

[codecov]

Codecov Report

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

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