chore: prepare v2.0.0

[janisto]

Pull Request

Why

The merged v2 work is still listed as unreleased even though the Go module already uses its /v2 path. Tagging from that state would leave the changelog and comparison links describing the release incorrectly.

What changed

• Kept an empty Unreleased section for subsequent work.
• Promoted the existing v2 migration and change notes into 2.0.0, dated 2026-07-22.
• Updated comparison links to use v2.0.0.

No runtime code, API, dependency, or module metadata changed.

Validation

• just qa: Passed; actionlint, zizmor, formatting, lint, build, package/example tests, and race tests were green.
• git diff --check: Passed.
• Additional checks: just vuln passed with an isolated Go build cache; no vulnerabilities were found.

Compatibility and risk

• Compatibility impact: None introduced by this changelog-only PR; the existing intentional v2 breaking changes remain documented in the migration section.
• Security and privacy impact: None.
• Failure mode and remaining risk: Incorrect release headings or comparison bases would make the published history misleading. If publication moves past 2026-07-22, update the release date before tagging.

Documentation and changelog

CHANGELOG.md now has an empty Unreleased section and a dated 2.0.0 section. README, examples, and API documentation were already aligned with v2 and did not require changes.