Clarify lazy runtime Close concurrency semantics

[apstndb]

Summary

• Clarifies the conservative LazyRuntime/LazyEmulator invariant for concurrent first Get/Setup and Close, including the Get-wins and Close-wins outcomes.
• Adds race-detector-friendly lazy state tests for Close-before-start, concurrent Get-and-Close, and Close after startup panic.

Fixes #39.

Test plan

• go test -race -run 'TestLazy' ./...
• TESTCONTAINERS_RYUK_DISABLED=true go test -race ./...
• git diff --check

Made with Cursor

[gemini-code-assist]

Code Review

This pull request updates the documentation for LazyEmulator and LazyRuntime to clarify their concurrent lifecycle invariants, specifically detailing how concurrent initialization (Get/Setup) and cleanup (Close) are serialized. Additionally, it introduces new unit tests in lazy_test.go to verify these behaviors, including edge cases like closing after a panic, closing before initialization, and concurrent get and close operations. I have no feedback to provide.

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