docs: complete post-v0.6.5 release notes

[tisonkun]

Summary

• audit user-visible changes between v0.6.5 and current main (263582e)
• add the missing oneshot progress fix to the Unreleased changelog and link the remaining entries to their source PRs
• describe the wake-outside-lock fix in terms of the affected public primitives
• correct the README and crate-level thread-safety claim so it reflects conditional Send and Sync bounds

Audit scope

The audit covered every commit plus the source and regression-test diff in v0.6.5..263582e. CI, benchmark scaffolding, internal refactors, and documentation-only clarification of existing Barrier cancellation behavior are intentionally excluded from the changelog because they do not change end-user behavior. The Unreleased heading is retained for the release workflow.

Validation

• cargo x lint
• cargo x test
• cargo x semver --release-version 0.6.6
• git diff --check