Runtime: coordinate GC, JIT, Wasm, and worker CPU budgets

[chrisbbreuer]

Parent: #459
Depends on: #476, #481, #482, #490
Related: #477, #478, #492, #503

Goal

Coordinate GC helpers, JIT/Wasm compilation, JavaScript Threads, Workers, waiters, and host tasks under explicit bounded CPU, memory, and latency budgets.

Problem

GC, native compilation, Wasm work, shared-realm Threads, isolated Workers, and benchmark workers make separate concurrency decisions. As parallel tiers land, independent pools/helpers can oversubscribe cores, steal latency from mutators, inflate RSS, or create priority inversions even when each subsystem is locally correct.

Scope

• Inventory every engine-created thread/helper/pool, its ownership, lifecycle, blocking behavior, memory/scratch, affinity, priority, and participation in GC/JIT teardown.
• Add a per-VM/process resource coordinator with configurable and automatically bounded CPU slots, compilation queue, GC helper count, worker admission, scratch/memory pressure, and shutdown.
• Prioritize mutator progress, foreground latency, termination/debugger work, GC safety, and bounded memory over speculative compilation.
• Prevent GC/JIT/Wasm helpers from deadlocking or starving when JavaScript threads block in Lock/Condition/Atomics.wait, host callbacks, join, or module loading.
• Apply backpressure/cancellation to stale compilation and queued internal work with exact ownership and no partially published artifact.
• Expose embedder controls and machine-readable scheduling decisions without making host thread handles cross-affine.
• Benchmark dedicated and oversubscribed hosts, small/large core counts, mixed subsystems, and memory pressure.

No-workaround rules

• No hardcoded benchmark core count, sleep/yield loop, hidden GIL, unbounded pool/queue, OS-priority abuse, dropped task, canceled required GC, or throughput result that silently starves latency work.

Acceptance

• A machine-readable thread/resource inventory has zero unclassified engine-created concurrency.
• Deterministic scheduler-pressure tests cover GC, compilation, Workers/Threads, Atomics/waits, modules, termination, OOM, and teardown.
• Perf: add causal attribution and exact-parent regression infrastructure #461/Perf: gate long-running latency, memory plateau, and cleanup #492 show bounded queues/threads/RSS, stable tail latency, and no throughput collapse under oversubscription.
• Single-thread/small-core workloads do not pay material coordination overhead.
• No-GIL, TSan, lifecycle/mid-GC fuzz, artifact lifetime, and Wasm threads gates pass.

Foundation

Current pooling/scheduler guidance lives in docs/threads/production-readiness.md and docs/threads/limits.md; subsystem work is tracked by #476/#481/#482/#490.