gc: add telemetry and dirty-page remembered scanning

[andrewtdiz]

Summary

• add PERRY_GC_TRACE JSONL telemetry for completed GC cycles, including phase timings, arena/malloc stats, remembered-set shape, write-barrier counters, evacuation, block persistence, and sweep/reset stats
• add arena range side metadata for fast nursery/old/longlived generation classification and use it in write barriers without per-page classifier-table RSS growth
• add slot-aware and external-slot dirty-page remembered tracking, old-page -> old-object indexing, and range-bounded dirty slot scanning for large arrays/objects/maps/closures
• wire runtime object/map helper stores through the barrier path and add tests for metadata lifecycle, dirty-page scanning, external map pages, slot-range bounds, old-map test fixture cleanup, and initialized raw closure fixtures used by conservative-stack tests

Verification

• cargo fmt --check
• cargo test -p perry-runtime -> 280 passed
• cargo test --release -p perry-runtime -> 280 passed
• for i in 1 2 3 4 5; do cargo test -p perry-runtime gc::tests; done -> all 5 passed
• scripts/run_memory_stability_tests.sh -> 18 pass, 0 fail; test_memory_json_churn.ts was 232 MB / 250 MB locally after compacting generation metadata
• cargo check -p perry-codegen
• cargo build --release -p perry
• git diff --check HEAD
• PERRY_WRITE_BARRIERS=1 PERRY_GC_TRACE=1 ./bench_json_roundtrip emitted 2 JSONL GC lines; jq -s -e confirmed telemetry field types
• no-trace ./bench_json_roundtrip produced 0 bytes on stderr