Prepare release 2.4.0

[cdcavell]

Summary

• bump shared assembly and template package version metadata to 2.4.0
• update NuGet package release notes for the minor release
• update README, package README, and template-packaging examples to 2.4.0
• update citation and Zenodo metadata with the July 20, 2026 release date
• preserve the existing package ID, template identity, short name, and supported template options

Release focus

Release 2.4.0 captures the accumulated backward-compatible feature and security work since 2.3.1, including:

• authenticated-by-default routed endpoints with an explicit no-auth opt-out
• secure authentication-cookie defaults and forwarded-header trust diagnostics
• framework-neutral audit accountability context and privacy-safe canonical mutation manifests
• opt-in audited transaction coordination
• durable audit-completion outbox and dispatcher
• audit reconciliation, integrity health checks, remediation evidence, and operational metrics
• DbContext-specific audit state isolation

Validation

• confirmed the branch contains only the seven intended release metadata and documentation files
• confirmed CHANGELOG.md is updated
• repository CI should validate version consistency, package generation, build, tests, formatting, documentation, template smoke tests, and release workflow contracts