[v0.4] RT-04: blank only test items, never the code after a cfg(test) that is not on an item (follow-up to #156) #169

Follow-up to #156 (review comment of 2026-10-17, #156 (comment)). without_test_items (crates/conformance/src/cases/runtime.rs:582-636) assumes every #[cfg(test)] is on an item. It ends the item at the first ; or { at nesting 0 after the attribute, and does not stop at a } that closes the enclosing scope. It also blanks the whole file when #![cfg(test)] appears anywhere. A runtime text after a struct with a #[cfg(test)] field, after an enum with a #[cfg(test)] variant, inside a function with a #[cfg(test)] parameter, in a match arm after a #[cfg(test)] arm, or anywhere in a file holding mod m { #![cfg(test)] }, passes conformance_rt_04. Nothing is hidden on main today: every #[cfg(test)] in crates/lexlean/src is on a mod.

Definition of done:

• A #[cfg(test)] blanks only when, after further outer attributes and a visibility, it is followed by an item keyword (mod, fn, use, impl, const, static, struct, enum, type, trait, macro_rules!). Otherwise nothing is blanked, and the code is read.
• The scan for an item's end stops at a } or , at nesting 0 that closes the enclosing scope.
• #![cfg(test)] blanks a file only as one of its leading inner attributes, before any item.
• rt_04 plants each shape in memory (field, variant, parameter, match arm, inline mod with #![cfg(test)]), each followed by a runtime text, and asserts each is found once.
• One shape is planted on the committed tree and recorded in VERIFICATION.md with its output.
• The base tree still reports only backend/core_v2.rs. No new row, no new diagnostic code, just vv green.