Disable Atomics agent suspension by default

[sebastienros]

Summary

• disable synchronous Atomics.wait suspension by default and preserve an explicit AgentCanSuspend = true compatibility opt-in
• keep Atomics.waitAsync available and preserve blocking Test262 worker agents while honoring CanBlockIsFalse
• document the breaking security default and server-thread denial guidance in the README and threat model
• retain waiter cleanup and GC coverage, including rejected waits, timeout/notify races, repeated waits, and worker harness behavior

Security behavior

A default engine now rejects Atomics.wait with a sanitized JavaScript TypeError before registering a waiter. Worker-like hosts may explicitly opt in, accepting that an indefinite synchronous wait cannot be preempted safely in process. Atomics.waitAsync remains nonblocking and unaffected.

The change was transplanted from the previously reviewed Atomics-only commit series without reimplementation. It applied conflict-free over the live sebros/default-stack-guard tip, preserving all prior security layers.

Validation

• Release multi-target Jint/Jint.csproj build: passed (0 warnings/errors)
• focused Atomics/waiter/Test262-agent tests: 12 passed
• full Jint.Tests under UTC: 5,800 passed, 4 skipped
• full Jint.Tests.PublicInterface under UTC: 1,737 passed, 9 skipped
• full Jint.Tests.PublicInterface with host-contract verification under UTC: 1,741 passed, 5 skipped
• git diff --check: passed
• security specialist review against origin/sebros/default-stack-guard: clean, no confirmed findings

The mandated NuGet proxy does not contain Meziantou.Analyzer 3.0.156 (nearest 3.0.141), so validation used a temporary analyzer-only downgrade to 3.0.141; Directory.Packages.props was restored exactly before publishing. The proxy also does not contain Test262Harness.Console 1.1.2, so the Test262 Atomics selection could not generate; the in-repo worker-agent harness paths were validated by the focused and full core suites.

Stack

#3057 -> #3056 -> #3054 -> #3052 -> #3051 -> #3046 -> #3045 -> #3037 -> #3036 -> #3035 -> #3030

[lahma]

Reviewed as the incremental diff against sebros/default-stack-guard. Straightforward and correct: flipping AgentCanSuspend to false makes a default engine behave like a browser main thread — Atomics.wait throws TypeError before a waiter is ever registered (per the spec's AgentCanSuspend check), Atomics.waitAsync is untouched, and the worker-like opt-in is one property. The Test262 harness change is right (worker agents suspend, CanBlockIsFalse tests don't, and spawned $262.agent engines opt in explicitly), and the new default-rejection test is unusually careful — the notify-loop cleanup path that prevents a regressed implementation from stranding an indefinite background waiter in the test process is a nice piece of defensive test engineering. Approving.

One note: breaking-default number five — same major-release migration-story point as the earlier flips; the README migration note added here is exactly the right shape for the combined release notes.