fix(gc): isolate class heap registries per agent (#8001)

[proggeramlug]

Summary

Isolates class-registry entries that contain heap pointers, NaN-boxed values, closure identities, or realm-created Symbol keys per Perry agent. This prevents a later perry/thread agent from reusing another realm heap values and prevents one moving collector from rewriting another agent roots.

Changes

• Move pointer/value-bearing class side tables to Perry hot thread-local storage.
• Keep process-wide code metadata such as vtables, method function pointers, bind lengths, registered class ids, and the synthetic id allocator global.
• Make both full and incremental GC root scanners visit and rewrite only the current agent class roots.
• Add a two-live-agent Rust regression that proves each agent materializes a distinct non-null Class.prototype.
• Add a perry/thread behavioral probe covering prototype mutation isolation in both directions.

Related issue

Closes #8001

Test plan

• cargo build --release clean (not run for the full workspace)
• Full affected-crate suite (targeted runtime regression and compile checks run instead)
• cargo test --release -p perry-runtime a_second_agents_declared_prototype_address_is_its_own -- --nocapture
• cargo check -p perry-runtime --tests
• cargo fmt --all -- --check
• parity filter 8001: 1/1 expected-output tests passed
• scripts/check_file_size.sh
• scripts/global_sink_isolation.py
• scripts/gc_runtime_root_holders.py
• Added runtime and test-files regression coverage
• Docs not needed: internal runtime ownership fix with no API change

The repository quick pre-tag suite otherwise passed, but its GC store-site inventory reports an existing marker omission at crates/perry-codegen/src/expr/property_set.rs:1457. That file is unchanged by this PR.

Screenshots / output

The new parity probe prints:

main: main
spawn agent: undefined/agent/true match: true
main after: main

Checklist

• I have NOT bumped the workspace version or edited CLAUDE.md / CHANGELOG.md
• My commit follows the fix: prefix convention
• I have read CONTRIBUTING.md and agree to the Code of Conduct

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📥 Commits

Reviewing files that changed from the base of the PR and between 8d1e456 and b2842e5.

📒 Files selected for processing (12)

• changelog.d/8001-agent-local-class-registries.md
• crates/perry-runtime/src/object/class_gc_roots.rs
• crates/perry-runtime/src/object/class_registry/construct.rs
• crates/perry-runtime/src/object/class_registry/gc_roots.rs
• crates/perry-runtime/src/object/class_registry/parent_static.rs
• crates/perry-runtime/src/object/class_registry/prototype_methods.rs
• crates/perry-runtime/src/object/class_registry/prototype_objects.rs
• crates/perry-runtime/src/object/class_registry/state.rs
• crates/perry-runtime/src/object/object_ops/descriptor_helpers.rs
• crates/perry-runtime/src/proxy/metadata.rs
• test-files/test_issue_8001_thread_class_prototype.ts
• test-parity/expected/test_issue_8001_thread_class_prototype.txt

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[proggeramlug]

Audited commit b2842e5 against current main. The ownership split is correct: heap pointers, NaN-boxed values, closure identities, and realm Symbol keys are agent-local, while code-only metadata remains global. The full and incremental root paths now scan only the current agent. Independent verification: release two-agent prototype regression passed; class side-table mark/rewrite, copying-minor value/key rewrite, Symbol-key rewrite, inheritance-root rewrite, and foreign-clear isolation regressions all passed. global_sink_isolation.py, gc_runtime_root_holders.py, check_file_size.sh, diff check, and current-main merge-tree check are clean. No blocking findings.