fix(ILO-460): reject nested fn decls with hint to use lambda or top-level helper

[danieljohnmorris]

Summary

• A name params>type;body declaration written inside another function's body used to silently hoist to top-level, losing any reference to an enclosing local (the rows case from the type-coerce-records feedback). Emit a precise parse-time diagnostic instead.
• New diagnostic ILO-P023 fires at the inner header, with a hint naming both rewrites: an inline lambda for one-off captures (proc = (x:n>n; +x rows) / {x> +x rows}), or a top-level helper that takes the captured value as an explicit param.
• Only fires when the inner decl is NOT separated from the surrounding body by an un-indented newline (decl_boundary), so genuinely-sibling top-level decls on separate physical lines keep parsing as before.
• SPEC.md gets a row in the cross-language gotchas table plus a one-line note in the function-decl section. ai.txt regenerated automatically by build.rs.

Follow-up

Closure-capturing nested fn decls (the substantive language feature alternative) tracked separately as ILO-485: https://linear.app/ilo-lang/issue/ILO-485

Closes ILO-460

Test plan

• cargo test --features cranelift,http,golden --lib — 3508 pass / 0 fail / 48 ignored
• New regression tests: nested_fn_decl_in_body_rejected, top_level_fn_decls_still_parse, inline_lambda_in_body_still_parses
• Existing top-level fn-decl tests continue to pass
• Inline lambdas inside fn bodies continue to parse

Generated with Claude Code

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
3804	1	3803	0

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

ilo::coverage_interpreter::mapr_short_circuits_on_err

Stack Traces | 0.013s run time

thread 'mapr_short_circuits_on_err' (42034) panicked at tests/coverage_interpreter.rs:950:5:
stderr={"code":"ILO-P024","labels":[{"col":49,"end":52,"line":1,"message":"here","primary":true,"start":48}],"message":"fn declarations are top-level only; this one is inside another function's body","notes":[],"severity":"error","suggestion":"use an inline lambda for a one-off helper that captures locals (e.g. `proc = (x:n>n; +x rows)` or `proc = {x> +x rows}`), or lift the helper to the top level and pass the captured value as an explicit parameter"}
{"code":"ILO-T005","labels":[{"col":64,"end":79,"line":1,"message":"","primary":true,"start":63}],"message":"undefined function 'f': its definition failed to parse","notes":["in function 'main'"],"severity":"error","suggestion":"fix the parse error first (see ILO-P024 reported earlier in this file); other references to 'f' are suppressed until then"}

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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