chore(gc): split barrier.rs at the 2000-line cap

[proggeramlug]

The evening merge round pushed gc/barrier.rs to 2,157 lines, going red on the file-size gate. Same remedy as #7830: the remembered-set inspection/drain/clear group (229 lines) moves to gc/barrier/maintenance.rs as a sibling module with explicit re-export — no logic change, verified by the full runtime suite (2,118 green single-threaded), the barrier-filtered suite, fmt, the file-size gate (green again), the thread-local policy gate (two cold entries re-keyed to the new path via --update, diff is pure rename), the root-holders gate, and the full local lint gate 19/19.

One resolution note: the moved group's pub(super) items widen to pub(in crate::gc) because a re-export cannot widen visibility — the reachable set is unchanged (same consumers in gc/cycle.rs and gc/fromspace_scan.rs that could already see them when barrier was a flat file).

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📥 Commits

Reviewing files that changed from the base of the PR and between 06ab783 and b34e430.

📒 Files selected for processing (4)

• changelog.d/7863-barrier-file-split.md
• crates/perry-runtime/src/gc/barrier/maintenance.rs
• crates/perry-runtime/src/gc/barrier/mod.rs
• scripts/thread_local_cold_allowlist.json

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