[Foundation] FT-6: Make logging thread-safe and per-run scoped (prereq for parallel execution)

[ChrisonSimtian]

Summary

Make logging thread-safe and per-run scoped, so concurrent target execution (FT-8) is safe. Prerequisite for parallelism.

Findings (file:line)

• Logging.cs:214 — InMemorySink.Instance with an unlocked List<LogEvent> (_logEvents); concurrent emits race.
• Logging.cs:239-243 — target-context enricher uses a static s_property (SetAndRestore), not safe under concurrent targets.
• Logging.cs:20 — global LevelSwitch; per-run level should live on the context.

Scope

• Lock or use a concurrent collection for InMemorySink.
• Replace the enricher's static s_property with AsyncLocal so per-target context is correct under concurrency.
• Per-run level switch held on BuildContext.Logging.

Acceptance criteria

• Concurrent emits don't race (stress test).
• Per-target log context is correct under parallel execution.

Notes

• Non-breaking.
• Depends on: FT-2. Prerequisite for: FT-8.
• Size: M.