Release Token Terminator 0.4.0

[AronAxe]

Summary

• promote the async/concurrency work from Unreleased to Token Terminator 0.4.0
• update README and migration instructions to the immutable v0.4.0 tag
• retry transient SQLite lock races during concurrent WAL initialization

Verification

• 142 passed, 2 skipped
• Ruff lint and format checks passed
• multi-process concurrency test passed 12 consecutive runs
• wheel and sdist built, Twine-checked, and containment-verified
• isolated installed-wheel Hermes smoke passed at 0.4.0
• structural benchmark: 86.23% aggregate token reduction, 0 provider calls
• independent AGY review: APPROVE