fix(runtime): restore hot TLS for arena right-sizing

[proggeramlug]

Summary

Follow-up to #9731, which landed through merge train #9735. The train was assembled before the final two reviewed branch commits, so it retained a raw thread_local! declaration and mislabeled decimal byte measurements as MiB.

• route arena right-sizing state through the runtime's hot-TLS cache
• label the reported decimal workload measurements as MB
• keep the landed right-sizing policy and behavior unchanged

Closes #9709.

Validation

• cargo fmt --all -- --check
• cargo check -p perry-runtime
• cargo test -p perry-runtime --lib gc::tests::arena_right_size -- --test-threads=1 (5 passed)
• cargo test -p perry-runtime --lib gc::tests::idle_reclaim -- --test-threads=1 (12 passed)
• python3 scripts/check_thread_locals.py --self-test
• policy checker before/after: gc/arena_right_size.rs is reported on landed main and absent with this patch; remaining reports are pre-existing files on main
• the underlying branch passed 3,104 runtime unit tests, focused arena/idle-reclaim tests, clippy, and all 62 applicable static lint gates before merge-train landing

Release checklist

• no version bump
• no Cargo.toml, Cargo.lock, CLAUDE.md, or CHANGELOG.md change

Summary by CodeRabbit

• Performance
  • Improved memory management for idle workloads by allowing arenas to reclaim excess capacity after sustained low utilization.
  • Right-sizing is bounded and remains paused until usage increases significantly, helping avoid repeated resizing.
  • Updated workload measurements to use MB instead of MiB for clearer capacity reporting.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Team

Run ID: 1e132fea-9fe6-47b7-8dbd-fdee41a3681d

📥 Commits

Reviewing files that changed from the base of the PR and between df36a5e and ec622b2.

📒 Files selected for processing (2)

• changelog.d/9731-arena-right-sizing.md
• crates/perry-runtime/src/gc/arena_right_size.rs

Included review availability: Your plan provides up to 8 included reviews per hour; 5 remain after this review.

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📝 Walkthrough

Walkthrough

The change switches arena right-size state declarations to the runtime thread-local macro and updates the changelog with MB-based measurements and revised capacity and RSS results.

Changes

Arena right-sizing

Layer / File(s)	Summary
Runtime wiring and measured results crates/perry-runtime/src/gc/arena_right_size.rs, changelog.d/9731-arena-right-sizing.md	The STATE and TEST_USAGE declarations now use crate::perry_thread_local!. The changelog reports updated MB-based workload measurements, arena capacity, and RSS values.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: ⚪ Minimal · up to ec622

This follow-up routes arena right-sizing state through the runtime TLS mechanism and corrects measurement labels to MB. No merge-readiness risk remains.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the primary change: restoring hot TLS for arena right-sizing.
Description check	✅ Passed	The description provides the change summary, related issue, validation commands, release constraints, and checklist information. It uses equivalent headings instead of the exact template headings, but…
Linked Issues check	✅ Passed	The PR preserves the arena right-sizing behavior requested by #9709 and applies the follow-up fixes for runtime hot TLS and accurate MB measurement labels. The provided validation confirms focused tes…
Out of Scope Changes check	✅ Passed	The changes are limited to arena right-sizing documentation, runtime hot-TLS declaration, and measurement-unit labeling. No unrelated code or repository metadata changes are present.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 1…

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[proggeramlug]

Landed via merge train #9737, together with three other commits my earlier trains missed the same way (#9704's and #9733's changelog fragments, and #9720's follow-up gap test). Thanks for catching this — the stale-head pick was my error.