Port GC checkpoint runtime work for #1090

[andrewtdiz]

Summary

This is the substantive runtime GC checkpoint toward #1090. It is intentionally separate from #1322, which only adds exact-head evidence tooling.

This PR ports the completed GC architecture work onto the split runtime tree and keeps the public GC facade/ABI intact while moving the collector toward the #1090 invariant:

minor GC cost = live young objects + dirty old pages/cards + mutable roots
not arena size + malloc table + old sweep + conservative stack guesses

What changed

• Split the old monolithic GC implementation into owner modules while preserving crate::gc::* and exported C ABI entrypoints.
• Added/ported mutable runtime root handling and copied-minor root rewriting coverage across shadow/global/runtime/FFI-style roots.
• Strengthened write-barrier/store-site coverage with dirty page/range tracking, external slot handling, store-site inventory checks, and helper-store tests.
• Added explicit GC type/layout metadata, pointer-layout/pointer-free scan accounting, typed shape descriptors, and early unboxed-object layout gates.
• Advanced copied-minor behavior: nursery reset, forwarding/rewrite verification, malloc interaction hardening, fallback telemetry, and survivor/promotion tests.
• Added old-page accounting/defrag groundwork and target collector architecture gates.
• Moved/covered small managed allocation paths and tightened runtime helper stores that keep young children alive across copied-minor GC.

Follow-up fix included in the port

While validating the port, I found and fixed a few real split/port gaps:

• malloc-backed closure capture writes now use the GC-container slot barrier path
• promise value / reason stores now route through GC-aware slot stores
• object overflow fields now dirty exact external slots
• copied-minor test guards reset runtime scanner/cache state so broad GC test runs are deterministic

Verification

Passed locally on branch codex/gc-1090-checkpoint-port-20260522 at 072055c42741f394ab8092c0ef51a1bd7a11a126:

• cargo fmt --check
• tests/test_gc_store_site_inventory.sh -> 188 files scanned
• cargo check -p perry-runtime
• cargo check -p perry-codegen
• cargo test -p perry-runtime --release runtime_roots -- --test-threads=1 -> 54 passed
• cargo test -p perry-runtime --release gc::tests -- --test-threads=1 -> 274 passed
• cargo test -p perry-codegen --test large_object_barriers -> 3 passed
• cargo test -p perry-codegen --test typed_shape_descriptors -> 12 passed
• cargo build --release -p perry
• scripts/run_memory_stability_tests.sh -> 60 PASS / 0 FAIL
• git diff --check

Memory stability highlights from the final run:

• test_memory_json_churn.ts default: 236 MB / 250 MB
• test_memory_json_churn.ts gen-gc-explicit: 236 MB / 250 MB
• test_memory_json_churn.ts force-evac+verify: 258 MB / 275 MB
• target collector architecture stress gates passed
• copied-minor fallback evidence report passed with top_remaining_reason=none
• low-pressure benchmark gates passed for 07_object_create, 12_binary_trees, and bench_gc_pressure

Scope

Refs #1090. This is a strong checkpoint toward #1090, not the final closing packet. The remaining closure work is full exact-head benchmark evidence, broader production policy hardening, and the rest of the long-term old-gen/typed-layout roadmap.