CI: `Warnings (product)` / `Warnings (host-compatible)` red on every PR — unused doc comment on a macro invocation in stack_maps.rs

[proggeramlug]

Summary

Both legs of the rustc-warnings gate (Warnings (product) and
Warnings (host-compatible)) fail on every PR, and on main:

error: unused doc comment
error: could not compile `perry-runtime` (lib) due to 1 previous error
##[error]Process completed with exit code 101.

The job runs with RUSTFLAGS: -D warnings, so the warning is an error there. It
is a plain cargo check warning everywhere else — visible in any local
cargo build -p perry-runtime.

Cause

crates/perry-runtime/src/gc/roots/stack_maps.rs:209 — a /// doc comment
directly above a macro invocation:

/// #7803 creation-cycle verifier (diagnostic, `PERRY_GC_NATIVE_SLOT_VERIFY=1`).
///
/// Runs a SECOND, non-rewriting native-slot walk after the rewrite passes of
/// ...
crate::perry_thread_local! {
    /// The rewrite walk's stats for the CURRENT cycle, ...
    static LAST_REWRITE_WALK: std::cell::Cell<(usize, usize, usize)> = ...;
}

rustc does not attach a doc comment to a macro invocation, so the outer block is
dropped and unused_doc_comments fires. (The inner one, on the static the
macro expands, is fine.) Landed with #8084 (53d63aad2).

Why it went unnoticed

The last test.yml run on main is 83b6b8c69 at 2026-08-15T04:22Z, where both
Warnings legs were green. #8084 merged at 12:25Z and there has been no main
run of that workflow since, so the breakage has only ever appeared on PR runs —
where it reads as "somebody else's red X".

Fix

Either demote the outer block to // line comments, or move it inside the macro
alongside the existing inner doc comment so it documents LAST_REWRITE_WALK.
The text is worth keeping — it explains the verifier — so moving it in is
probably the better of the two.

Noticed on #8165, where it is one of three red checks, all pre-existing; the
other two are #8170 and #8155.